Best use of my money? (CPU upgrade)

Your upgrade seems logical. The problem with upgrading your GPU is you will probably need a power supply upgrade as well. There is no problem with upgrading your PC in steps. Your can come back later and add a GPU and PSU.

I would switch to an Intel Core i3. You can get an i5 later on!

Good upgrade, but really a GPU upgrade will make the most difference when it comes to FPS... as avensmith said, you might have to upgrade your PSU to accommodate the new card under load.

Really I would get a GTX 960 2GB Asus Strix.

Yes the 960 is a good budget-ish card.

You won't need to get a better PSU to power a 960, those things sip on electricity. I have an 8320e and a 960 Strix running on a 430W psu.

You make sense. Unlike AMD fanboys.

The reason the SSD even has issue is because the MOBO is a SATA II (3Gb/s) not SATA II (6Gb/s) which SSDs need to optimally pump out all that 'speed' you invested into buying a SSD to begin with. So logically the MOBO needs to be replaced. Further your running PCI-E II slots which again for any GPU will limit the maximum output for that card, while compatible your not getting the 'performance' you paid for because again the MOBO needs to be replaced.

I would suggest replacing the MOBO first and foremost. Upgrading to FX8 seems logical but the issue is the FX line is long been abandoned and dead (SIGH!) and now even the most 'expensive' FX8 you could invest in would only match up ABOVE a Intel i3 and still WAY BELOW the performance of a i5 for " heavy gaming. quite frequently"

So spending for a new Mobo, Cooling AND CPU this way IMO is a waste because your shooting yourself in the foot immediately and wasting the money (there is no and will be nothing 'past' a FX8, as AMD has shifted to thinking Tablets and other 'portable' devices and mainly makes APUs for a long while now). I would suggest instead swapping to a iCore based Mobo and CPU (check for compatibility with your RAM too or like the PSU when you go to change GPU you will have to invest in THAT additional cost).

Now to toss in the 800LB Gorilla. Is this a self built PC? If not, if this was a HP, Dell, etc. then guess what - Windows won't work when you make any of these changes. Windows OEM editions are ONLY working with the make / model they are sold with, EVEN if you reinstall it, try to do a repair, whatever it will NOT allow it to work, no matter what you try, because when it starts it asks "what hardware do I have to work with today" and boom, that isn't what I was sold with - Wrong OS/Hardware Error - FAIL. So on top of those other costs your plunking down and have to REINSTALL CLEAN EDITION of Windows ($179 US) on top of this.

Considering all this, it MIGHT make better sense to either
A) Save up to buy later a proper system completely (usually $800US for a i5, Nvidia 970, etc. type, check www.pcpartpicker.com for pricing when you go to purchase as the prices change based on time of year)

B) Invest in a PS4 for your 'heavy gaming' now, as it can play anything that is 'sold' and alot of them 1080P, 30-60FPS depending on the game optimization on HIGH and you can get a Refurb PS4 for $200US, then work on a (A) above while still being able today to do after effects and Illustrator on what you got.

Get a PS4?!?!

1. PRICE, PRICE, PRICE (note OP title Best use of my money?).
There is no way to get a full PC (Cpu, Mobo, PSU, GPU) in parts capable to play better than a PS4 when your talking only 'couple hundred' to spend with "heavy gaming." Case in point GTA V, PS4 does it on HIGH graphics, 1080P @ 30FPS - NO full PC (Cpu, Mobo, PSU, GPU) in parts for 'couple hundred' can play it on HIGH AND 1080P. The parts necessary to do 'better' than 1080P 30FPS High immediate just the GPU is over $200 - boom no CPU, no Mobo, no PSU to POWER the GPU.

2. 95% of the poster on TH are Joe/Jane-Iain'tgotacluewhatPSUmeans. These are not DIY, techies, and repeatedly ask do they want to risk 'all that money' when you later post "missy my GF super hairycat jumped and rubbed on me insert that new CPU - is that why my new build doesn't start up?" - all these just want to go to Walmart, grab a box, plug it in like a toaster and it just 'makes it happen'. No risk, no extra cost, no 'techy', nothing. Just plug it in, use it, and like a toaster just 'works'.

3. NO RISK - as I pointed out any of the solutions we can come up with have big risk of messing up, simply buying a 'box' and plug it in solves 95% of the TH users here.

So yeah, get a PS4. It is CHEAP, it WORKS on EVERY GAME (they can't sell a game for PS4 that doesn't work on everyone's PS4, but you can't even close to say that for every PC out there), and does it well. AND YES even the new Fallout 4 will let you MOD your PS4 game as well, so for 95% out there, it just 'works'.

Console fanboys. Since when was a console better than a PC of the same price.

This is much better. Instead of an i5 you can get a Xeon. A Xeon E3 is an i7 without the iGPU. You get more performance and less cost.
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Xeon E3-1241 V3 3.5GHz Quad-Core Processor (£213.52 @ More Computers)
Motherboard: Asus H97-PLUS ATX LGA1150 Motherboard (£82.76 @ Scan.co.uk)
Total: £296.28
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-07-16 00:23 BST+0100
Xeons have HT which majorly helps in multi-threaded games, and rendering etc,etc. In games you will see a 10 fps increase going with the Xeon. The xeon has HT and the i5 doesnt.

I hate console fanboys. Maybe a console would be worth it if you could game in 1080p @60 FPS.

No Name calling first off.
Second off you don't know me, and if you did you seen all my other posts where I honestly say I don't do consoles because they hurt my hands (can't even use the controller on a PC). I love my K/M and been gaming since.. uhmm... 1984 on my Vic20, so yeah not a console fanboy.
I am honest, because I see independent testing results like this :
http://www.ign.com/wikis/xbox-one/PS4_vs._Xbox_One_Native_Resolutions_and_Framerates

Alien: Isolation 1080p @ 30fps [n]
Battlefield: Hardline 1080p @ 60fps [n]
Blacklight Retribution 1080p @ 60fps [n]
Call of Duty: Advanced Warfare 1080p @ 60fps(targeted) [n]
Call of Duty: Ghosts 1080p @ 60fps [n]
Dying Light 1080p @ 60fps [n]
Minecraft 1080p @ 60fps [n]
Planetside 2 1080p @ 60fps [n]
Tomb Raider: Definitive Edition 1080p @ 60fps [n] and so forth.

(and console ONLY games like The Last of Us: Remastered 1080p @ 60fps [n] - AARRGGGGG!!!)
And I have to admit the PS4 (NOT THE XBONEOne) does for the 95% out there what a " PC of the same price."

First let's exam what you get for a PS4:
http://www.gamestop.com/ps4/consoles/playstation-4-500gb-system-black-friday-bundle-with-grand-theft-auto-v-and-the-last-of-us-remastered/118074#productDetails
$400 PlayStation 4 500 GB System Grand Theft Auto V Bundle Includes:
PlayStation 4 System (which has a 3D Blu Ray Player)
DualShock 4 Wireless Controller
Grand Theft Auto V Game Disc
The Last of Us Remastered Download Voucher
Wired Mono Headset

HERE IS YOUR GOAL THEN: PC capable of playing HIGH GRAPHICS and get
The Last of Us: Remastered 1080p @ 60fps
GTA V1 1080p @ 30FPS

Case in point for OP here, we are talking $400 ....
So using the scores we see here
http://www.techspot.com/review/991-gta-5-pc-benchmarks/page6.html FX6350 69FPS
And here http://www.techspot.com/review/991-gta-5-pc-benchmarks/page2.html
Minimally a HD7950 or Geoforce 760 each @ 57FPS or better
so that we get a average around 60FPS (69FPS+ 57FPS = 126/2 = 63Est. FPS)
We will just add pcpartpicker.com parts and other sources.

http://pcpartpicker.com/p/gx2HTW Total: $498.01 and plays the titles at 1080P High @60FPS

We substitute State of Decay YOSE for Last of Us, we use a mono headset than the one with the PS4, increase to 16GB to give OP plenty for multitasking AND the extra room for heavy programs like Illustrator and After Effects, made sure a proper PSU, and provide Blu Ray and Wireless controller, which needs the add on free software to properly work on PC. The PS Wireless Dualshock was rated THE BEST CONTROLLER PC Magazine Apr 2015 for PCs.

Again though this does not fix using the old MOBO which chokes the bandwidth of the GPU / SDDs and doesn't include the cost to replace that Mobo and CPU. Those would again add another 200-300 over that price, which back to my initial statement "There is no way to get a full PC (Cpu, Mobo, PSU, GPU) in parts capable to play better than a PS4 when your talking only 'couple hundred' to spend with "heavy gaming." "

1. PRICE, PRICE, PRICE (note OP title Best use of my money?). ... PS/4
2. 95% of the poster on TH are Joe/Jane-Iain'tgotacluewhatPSUmeans. .... PS/4
3. NO RISK -... PS/4 - plug in and just play.

Love my PC but I got to admit this truth, on these critical points that sway 95% of the people here.

They come with coolers. If you are not OCing or something the stock coolers are fine.