Intel Core i3 vs i5 vs i7

nope keep the i3 till someone makes a better cpu. a i3 is fine for gaming

No, the system you have is very well matched. The H81 motherboard is the lowest end as is 1600 mhz memory. Best bet is to enjoy the excellent system you have now and start saving for a new system in a couple years.

Sorry, didnt notice your specs. I would switch to an i5 4460 and sell the i3, you will be out of pocket about 60 quid. No clocking, bargain basement board.

Yes, You can overclock a Pentium G3258 dual core. That would down grade from the excellent CPU that you have now. We all get the upgrade bug, but your system is perfectly balanced in my view. Maybe you could get a nice gaming mouse or keyboard or a new 27" IPS monitor if you do not have these yet. Your money though, do as you please.

C.E.X. warrant stuff for one year,replacement only. NEVER CASH BACK!!
but some of the techies are lying sh1ts. Ive bought a lot of ram etc over the last 6 years from tot crt rd/rathbone place and mostly I have had no issues. I bought 2 cpu's from them, one was dead and they refunded, they admitted not even tested it. DONT TRADE ANYTHING IN with them, they will rip you to pieces.
The best place for a used i5 is novatech's ebay page.
I bought my 4430 from them 2 years ago.60 day no quible return, £95. CEX have never been cheap,a new one is little more than they are asking.
If c.e.x. HAVE to give cash for a refund after the first month, it is at the current price they are selling for minus a portion for use had. ;#{

Much depends on the type of games you play.
Games such as sims, mmo, and strategy games depend on the performance of a single master core.
The i3-4170 @3.7 is very good for those types of games. The single thread passmark rating is 2136.
The total passmark rating is 5174.

By comparison, the i5-4590 has a single thread passmark rating of 2121 which is of no real help.
But, the total rating is better at 7214 when all 4 threads are in use.

Few games can effectively use more than 2-3 threads.

If your games are multiplayer with large numbers of participants, then a quad with a higher total rating can be helpful.

To help clarify your CPU/GPU options, run these two tests:

a) Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.

b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 30% improvement in core speed might do.

You should also experiment with removing one core. You can do this in the windows msconfig boot advanced options option. set the number of processors to less than you have.
This will tell you how sensitive your games are to the benefits of many cores.

If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.

It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.

You could opt for a I7-4790K which comes with a higher stock clock of 4.0 and 4.4 turbo.

A i5-4690K can be overclocked to those numbers and perhaps higher, but, you would want a z chipset motherboard.

Most likely, I think, your best gaming upgrade will be to a stronger graphics card.

I would have no problem buying used from a good seller on ebay.
Know your prices, pay only with paypal. test promptly.
ebay will side with a buyer if the product is claimed to be not as advertised.

Many individuals are selling used i7-4790K chips in order to upgrade to skylake and are not too concerned about the price they fetch. I did that myself.

its not a simple yes or no. it depends on the game some will run flawlessly between the 970 and i3 and some will be bottlenecked by the i3

From a graphics point of view, a GTX970 can handle most any game well at 1080P.

From a cpu point of view, the best choice is not entirely clear.
Since you already own a i3-4170, I suggest you buy the GTX970 first and then see how you do with YOUR games.
No doubt, it will be a nice improvement, particularly for first person shooters which tend to require good graphics.

If you determine that a cpu upgrade is in order, you will then know if you need faster individual cores, or if you need more cores.

I think only a i7-4790K running at stock is likely to be a good upgrade for your current motherboard.

Nobody but YOU knows what games will come out in two years, what their requirements are, and which ones YOU will favor.
There is really no such thing as "future proofing"
Buy what you need today.... today.
When the time comes that your current pc does not do the job, plan on a suitable upgrade