Prior to upgrading to a 6850, get a decent PSU first. And yes, your processor might drag any video card no less than a hd6750/hd5750 or gts450. BUT, to be able to match the gfx power of an hd6770 or even a 6850, you can overclock your processor to 11x333mhz = 3.667ghz (or even more if you're feeling brave

). This should be a walk in the park and definitely worry-free since 1333fsb should be native to your board. There might even be no need to up the vcore.
Naa ba diay clock/voltage/timing options imong board?