I'm gonna make the assumption that the link you posted below is the desktop your talking about. Anyways, I believe TF2 and MW3 will definitely run at max settings on your desktop, and BF3 will probably run at High settings (BF3 has a setting for ultra as well). And yea you should probably be getting 60 fps+ as well (at 1080p, idk if your gonna try running higher than that).
As nadim615 posted earlier, you'll be maxxing out games like TF2 and MW3, but you'll definitely not be able to play BF3 on Ultra settings, you need to use Crossfire for AMD-based cards or SLi for Nvidia cards. Still, that game will run well above 40 FPS, so quite playable, on High, at 1920x1080.
the 6850 has a little less grunt than mine but has a stronger cpu. on my system i can play on high settings and average 60 fps on bf3, that system will average 58 or there abouts...
1.300. is a pretty reasonable price for a pre build, which would make me suspect of the quality of parts there using.