I had an X2 here that gave me 20340 points in 3dmark with a quad at 4ghz... pretty rocksolid, though take into consideration pretty hefty heat output in the case and requirements of PSU with one 6 pin and one 8 pin conector...

If you got an SLI board go 9800GTX if you already own one, though even that setup will be not as good as the X2 in Crysis... but for all the other games it will be plenty fast if ya can bundle it with a speedy CPU...