I upgraded from a 660TI to a 970 a few months back and I can highly recommend it. I did a fair bit of research at the time and found that the 970 was a slightly better choice than the 780TI and the 980 just wasn't worth the extra money.

 

The 970 is newer tech so should be a bit more future proofed. (I have a 600W PSU here too)

 

This is in no way a slight against 780TI owners, for me the 970 was the better choice.

This chart is always worth a look when shopping for GPUs

 

http://www.videocardbenchmark.net/high_end_gpus.html

 

Personally, unless you're thinking about going 4K, I can't see the justification for going beyond a 970. Heck, even with my lowly GTX 680, my rig is CPU bound 99.99% of the time.  Of course I tend to stick to fair weather flying, so even with 8xsgss, I seldom max out my GPU - YMMV.

The 970 is newer tech so should be a bit more future proofed.

 

Bizarrely that's the least convincing argument for me, as far as I'm aware the 970 has nothing new in terms of graphical rendering hardware and in terms of performance is significantly slower than the 780Ti. As far as I'm aware the newer tech just means it uses significantly less power, which is no bad thing but does not make for a better card.

Bizarrely that's the least convincing argument for me, as far as I'm aware the 970 has nothing new in terms of graphical rendering hardware and in terms of performance is significantly slower than the 780Ti. As far as I'm aware the newer tech just means it uses significantly less power, which is no bad thing but does not make for a better card.

 

Using less power is arguably more helpful for your entire system. 780Ti is Kepler and the 970 is Maxwell. I just said the newer tech should be a bit more future proofed.

The 970 is significantly better than the 660Ti I was upgrading, but I wouldn't say the 780Ti is significantly better than the 970. (it was significantly more expensive though )

 

Benchmarks are always a bit subjective but here's a good comparison http://gpuboss.com/gpus/GeForce-GTX-970-vs-GeForce-GTX-780-Ti

Also, a great tool for upgrading is toms hardware hierarchy charts. http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html They recommend only upgrading to a card that is at least 3 tiers higher than your current one. the 970 is three tiers higher than the 660Ti (Granted, the 780Ti is four tiers higher, but like I said I couldn't justify the price increase at the time)