There are differences between SLI and Crossfire.
Crossfire requires one card to be the master card, and one card that is identical to the master card, acts as a slave. They are then connected by a special video cord that connect to both cards and the DVI or VGA from your monitor.
With SLI, you also need Identical cards, and by that I mean, if you have one 7900GTX then you will need a second 7900GTX, only you dont have to have the same company for the card. Also, where Nvidia has ATi beat, is how the cards are connected. Nvidia has a slot on the side of each card that serves as the interconnect to both cards. Usually the motherboard manufacturer will ship a SLI Connector which is a proprietary connector to connect two SLI ready cards.
I have read recently that ATi plans to have the same feature and do away with the current crossfire setup. It remains to be seen, but it would fair much better.
Also, if you buy a motherboard and are looking for SLI, then usually you will see these chipsets or names on the motherboard,
Nforce 4 SLI
for example my motherboard: Biostar NF4SLI-A9
Newer Nforce 5xx series motherboards carrying the SLI acronymn.
Crossfire will generally be the ATi chipset of Express 3200.