Which upgrade for less than 600?

I had 4gb of ram go bad recently, out of my 8gb total, and while the manufacturer is willing to replace under warranty, it got my thinking. I built my pc in fall 2012, and realize I'm getting to the point where updating a few components should put me higher on the performance curve again (as in: my pc performed with the best of them 3 years ago, but is struggling with newer games).

Also of note, I went the AM3+ path, because I was expecting more from AMD in the desktop CPU market than they've actually produced(I took price over piledriver because the performance justification wasn't there, but they haven't updated since the fx-9590, which I'm considering as an upgrade, but I don't want to buy a new cpu only to have them change socket on me on the next desktop release)

Reason for updating: wanting faster pc for gaming and 3d editing/modeling (I'm a wannabe independent game developer)

Budget: less than 600 preferably, not including shipping/rebates.

Timeframe: by end of 2015

Preferred purchase method: Newegg.com or bestbuy.com
Overclocking: maybe mildly
SLI: potentially
Monitors: 2x Asus vs247HP, with a samsung s24c230bl sitting in a box, could potentially be added in the future. All are 1080p monitors

Current Setup:

Case: CM 690 II advanced case with stock fans plus a few extra cm blade master 120's added(have spare bm 120's as well)

Mobo: Asus Sabertooth 990FX gen1(not the R2, etc) AM3+ socketed

CPU: AMD FX4170 quadcore

CPU cooling: CM Hyper 212 EVO with 2fan push/pull setup

Ram: Team Dark 8gb (2x4gb) DDR3 1600 (came with the mobo as a packaged deal, currently only 1 stick is working though)

PSU: CORSAIR Enthusiast Series TX750 V2 750W ATX12V v2.31/ EPS12V v2.92 80 PLUS BRONZE Certified Active PFC High Performance

GPU: MSI 660ti 2gb OC edition

Storage: ocz vertex 4 128gb , plus a used 200gb and 80gb HDD from other pc's
And a DVD burner, as well.

*************Considered upgrade paths*************

I REALLY don't want to replace the Mobo at this time, so no "just swap mobos and go intel" please. I am considering swapping to a liquid cooled fx-9590, but that's 2 year old tech, and I'm also kind of holding out in hopes that AMD will pull their head from dark and remote places, and release a newer chip on the AM3+ socket. Also of note, I spent months researching the AMD cpu's etc, but know next to nothing about intels products(naming conventions, etc.). Not saying a swap isn't completely out of the question, but it's my least desirable option at this time.

I have some thermal space available, so considering either a second 660ti in sli(could be had for less than 200 used from a reliable dealer on ebay), or upgrading to a single gtx 970 OR 2x gtx 960 in sli(less than desirable, since from what I can tell a second 660 ti would have better performance than 2 960's in sli), or if I'm doing no other upgrades, a single gtx 980(not ti until the price drops). Problem is, I get the feeling that I'm going to bottleneck on my cpu while upgrading the gpu,

I'm also considering upgrading to either 16 or 32gb of ram, and adding another SSD and HDD(not much room for apps after the windows 7 installation on the 128gb)

Basically I'm trying to figure out what will give me the most bang for my buck, though I prefer to NOT upgrade with tech that is already dated(such as the fx-9590), because I want any upgrades I make to make the pc last another 3+ years. I'm also fully aware that I could put together a Haswell-E machine(barebones, and likely using the GTX 660ti from current machine) for just a few hundred more, and turn this pc into a home server.

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I know I've probably rambled through this a bit, and I now have more questions than when I started typing this out. I guess my biggest question is, does my am3+ socketed pc still have room to grow, or am I essentially topped out techwise, and in need of building a new pc(which would be disappointing, because I originally intended this to be a 5 year machine, and it feels like I'm got the short-end from the cpu manufacturer, since I originally based my plan on their released architecture schedule which they have NOT been keeping up with for desktop am3+ solutions).