Can i swap out an nvidia 9800 gt for an nvidia 260 gx

Sure. The only difference is the amount of power used and the manufacturers minimum power requirements is likely overstated.
You may want to consider an HD5770 instead however. It offers similar performance while using less power, offers DX11 compatibility and 3 monitor support. It's also usually slightly cheaper these days.

ok well im looking to play games like modern warfare 2 and left for dead 2 on max. the 5570 is manufactured by ati correct? would it be compatible with my motherboard?

You can definitely use the HD5770. You said your board has two PCIe x16 slots so the only issue is whether it is SLI, Crossfire or both. If it is only SLI compatible then you wont be able to use two ATI cards at once.

well i read reviews for the pc online prior to purchasing it. people were talking of swapping out the gt and going crossfire...so im sure its possible. what kind of a power supply would i need to run crossfire...and would it be the same watts as say dual nvidia 260 gtx's?  

Nah, the HD5770 uses much less power than the GTX 260. A quality 600w PSU should be fine for crossfiring them. For two GTX 260s I would probably want a 750w PSU.
The card also scales well in crossfire so 2 of them are close to the performance of an HD5870.