depends on your friend's budget, psu, display resolution, case - his system specs.
if your friend can, oc the athlon dual with a good cooler and try to unlock other cores.
radeon hd 6850 is a better gfx card than the 550ti. bf3 can be played on both cards. consult this article:http://www.tomshardware.com/reviews/battlefield-3-graphics-performance,3063.html