if it doesn't have dual pci-express x16 slots, dual cards are out of the question. for nvidia sli, you'd need a mobo based on an sli chipset. amd crossfire works best with ati or intel chipsets.
if your mobo can support dual cards, then you need to decide on your budget. 2 geforce 9600gt cards ($280) is faster than a 9800gtx (also $280) in some games but slower in others.
any cards below the 9600gt aren't worth sli since they'll be outperformed by a single 9600gt. likewise, crossfired ati radeons are generally outperformed by their nvidia counterparts at the same price point.
Lately I've decided i wanted to upgrade my computer, and seeing as how my CPU, amount of RAM, and pretty much all other components are fine, I looked to my seemingly ancient 7800GTX 256mb video card, and wondered how i should upgrade it. I've been reading around, but I remain confused as to whether it is more cost/performance efficient to get two 512 mb cards or a single 1 gb card. I'm not sure how big a factor memory alone is to how good a video card is, and am wondering to what extent, if any if the memory is the same, 2 GPU's would have an advantage over 1. My confusion arose out of that 2 512 cards would have the same amount of memory as memory as a 1 gb card, and yet a 1 gb card would seem to be cheaper. For that matter, would there be any other better solution to this, generally my budget for this would be around 400-600 dollars.