I'm building a new system on a budget (~$800 not including the monitor or OS) and I can't decide between an AMD FX-6300 with an HD 7870, or an i5-3500k with an HD 7850. I'll mostly be using this computer for gaming, but I also want it to last for a few years. I think I'd like to wait at least a year and a half before upgrading my graphics card.
If it were me, i'd go with the i5 and HD 7850. If I remember correctly the i5 is significantly better than the FX6300. The HD7850 should be able to handle most games pretty well, and you can always upgrade the video card one year from now and still have a pretty damn good CPU.