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.