[quote name='CaseyRyback']crossfire is the same thing, though it is done with ATi cards. The difference is that ATi uses a master and slave card where the master card is a special card and the second card can be any brand or any speed based on the model of master card you buy. As far as Nvidia goes, unless something changed (there was talk of this happening) basically you had to have two identical cards though the benefit is that there is no special card to run SLI and the mobo's are cheaper.
IMO you should bypass crossfire and SLI and just buy a high end card because the cost/benefit ratio is not worth it. And even then as long as you don't need one soon you should probably wait for the directX10 cards coming out later in the year so that you can use all of the new features that games are going to be putting out in the coming months.[/QUOTE]
I asked about this recently at the Cnet forums and was told the same thing... while two mid-range cards can handle some things faster, a single high end card can do a lot more and last longer. The problem is that I can't seem to find any comparisons between a single card (say 6600GT) and that same card in SLI (two 6600GTs).
And yeah, in the end I'd just wait for DX10 since you'll need one to fully take advantage of Vista (if you plan to upgrade).