damric :
The 280X would be a noticeable upgrade, but you really need a durable power supply to run that card, not some generic.
I think you should look at the GTX 950. It's about 3x as fast as your 6870, but has low power draw. You can use it without upgrading your PSU.
Another option is the R9 285 aka R9 380. Those use a bit more power but not as much as the 280X.
Oh no, my power supply is not generic! I opened to check and it's Corsair CX500.
But hey, I always thought I couldn't mix AMD CPU with Geforce. That's why I didn't even mention it here, sorry for that.
If this comparison is correct
http://gpuboss.com/gpus/Radeon-R9-280X-vs-GeForce-GTX-950
Between them, I guess the GTX950 is actually the best option, even if my PSU can handle the 280x. It's ~$60 cheaper!
Do you think it's worth buying the R9 280x, or perhaps GTX660, anyway? I'm a complete newbie, but they don't seem to do much better in these tests to cost so much more.
Also should I look for OC, SC, SSC, the price difference it worth? Or can I just overclock later and get the same results (I know how)?
These different versions are so confusing, MSI, EVGA, Gigabyte, etc.
Thanks a lot for your help, I'm almost there.