Originally Posted by
Incognito Noctix
I would also go for a GTX 670, specifically the Gigabyte GeForce GTX 670 WindForce 3X. It has excellent reviews.
If you want AMD, you'll be looking at the 7970, or 7950. I can't really recommend a specific one there though.
Also, you shouldn't have to change anything. Depending on your motherboard, it might not have PCI-E 3, but it will still work perfectly. Some would argue you wouldn't see much of a performance boost with a PCI-E 3 motherboard anyway, but I can't comment on that.