Honestly, i'd wait, there are so many things that will be coming out from now till like may this year. lower 7k amd graphics card, nvidias new gfx cards, piledriver(trinity as well), ivy bridge, windows 8.
Right now you can already build a 400-450$ gaming comp using a a8-3870+6670(both of them OC'd) that will play bf3 on a mix of med/high settings with 1080p res, so people have high hopes for trinity.
If you absolutely can't wait, get an intel system if you want to sli or crossfire graphics card, otherwise amd will be cheaper and wont bottleneck a single gpu(that's if you can get your hands on a nice phenom II x4 BE).
The AM3+ socket will support piledriver and the current sandy bridge mobos will support ivy bridge.
Edit: Also mentioning your budget amount + if you already have stuff like a harddrive, disk reader, case, psu etc will be helpful.