Difficult to compare different video cards especialy ati and nvidea, only thing for sure is whats top of the line right now will be out classed by something else in six months if not sooner so avoid very expensive bleeding edge technology unless youve money to burn, best bet is to look through reviews on sites like bit tech, anand tech and toms hardware that cater to gamers, as for if it will work in your system, if it will fit it will work without probs 99.99% of the time if the drivers are up to date, if theres a problem send it back straight away.
Q6600-P5K Premium+Alphacool w/blocks
EVGA 8800gt+Koolance w/block
Samsung f1 raid0 +f2 hard drives
Enermax infinity 650w Popeye67's Blog