As for the GRAPHICS CARD, it's a matter of budget.
The HD6870 would still look fine and would be minimum recommend (and can be found on sale at Newegg for $140 after $20 MIR, or NCIX for $150 after MIR).
The GTX680 is $500 but with the i5-2500K will basically max all quality settings at 1920x1080 and still give you 60FPS.
The GTX670 should be $400 and out in a month or so but availability on the new GTX6xx's is limited currently.
I've got an HD5870 which is pretty close to an HD6870. There are plenty of games I can run at full, or close to full settings, however I'm still upgrading to the GTX680. The main games I'm upgrading for are:
- Witcher 2
- Metro 2033
- Batman AC
Many of my games that get 60FPS most of the time will still be more enjoyable due to less stutter and reduced sluggishness caused by VSYNC.
*Anyway, check the benchmarks. It's worth considering that an HD6870 for $150 should still sell for at least $80 after a year and of course the i5-2500K is a strong base for upgrading.
PCIe v3??
If it were considerably more expensive, I would not recommend it. However there are nice Sandy Bridge and Ivy Bridge motherboards that do support it for only $150.
http://us.ncix.com/products/?sku=69231&vpn=Z77A-G45&manufacture=MSI%2FMicroStar&promoid=1259
I believe the above is a new Ivy Bridge motherboard. It supports the previous 1155 CPU's as well as the newer ones. This is a strong base to build a system around. I haven't compared this board extensively, however I don't think you can go wrong here.