lancer said:
so you are saying that you don't need specific drivers on a SLI setup for each game you play?
SLI has a config file that tells the SLI cards how to work with each game. Nvidia updates the files (included with their driver updates) for most new games. Older games you may have to build your own file for. There is a thread around here somewhere with a link to instructions on how to edit your own config file.
Crossfire offers more ways of splitting the work load between the two video cards which makes it more friendly to a broader range of games. It just hasn't been around as long as SLI so the experience base is thin and now the improved 3200 crossfire chip is out but only available on one MB so far and I haven't seen any performance comparison reviews.
Some other things to think about.
"Many" newer ATI cards are crossfire compatible as slave cards, except the AIW cards which are not compatible (something they don't mention up front). Also, call ATI and verify that the specific card you are looking at is compatible and has a compatible "master" card. Crossfire is supposed to be much more mix and match friendly (with llimitations) than SLI cards are but I no longer take any claims like that at face value (from anybody, ATI, Nvidia, video, MB, etc.).
Another issue is heat. This year Nvidia cards are running cooler than ATI so single slot options are much more readily available.
Gigabyte (and maybe ASUS?) have dual SLI cards in one pack now.
Quad cards are now being sold (SAZARs new subsidiary Alienware has one).
Dual vs Single:
I did a cost benefit analysis a few weeks back between dual card and single card video rigs and I think it was pretty much a wash on cost. Taking into account PSU upgrades, extra $ for dual video capable MB and video cards over single video card versions, the price drop on a given card over the time it takes your first card to become too slow for the latest games.
That left secondary considerations as the deciding factor:
-noise level (2 GPU fans make more noise than 1),
-space (1 slot or 2-4 slots used up),
-waste heat that needs to be airconditioned,
-complexity/risk (making 2 high performance video cards play well together is harder than one card).
-trouble shooting as a card starts getting flaky (which to RMA?)
-will next years card support important new shading features that aren't in my old card? (That means you can upgrade raw speed but not image quality.)
-being able to drop a $60 6600GT into your rig and playing new games at high speed vs buying a $300+ next gen card.
Single or dual video is a tough call and going to be a matter of personal preference.