umm...do not SLi...save your money SLi isnt even needed on games yet...for the price of SLi you could buy a better video card that will out perform the 2x 6600GT....
if i was you i would just get a 512mb stick of ram and the 6600GT for now..then later down the track build a complete new system
if i was you i would just get a 512mb stick of ram and the 6600GT for now..then later down the track build a complete new system