Crossfire/SLI confusion

I’m a bit confused about SLI and Crossfire (maybe this could be added to Maziar’s sticky FAQ). Do SLI and Crossfire only come into play when multiple graphics cards are installed on a single board?

If I have a Crossfire-ready board (say an Asus P5E3 Deluxe) can I put a single SLI graphics card (f.i. MSI GeForce 8800GTX) in that board and run a single monitor? How about an SLI board with a Crossfire card and a single monitor? Are there any drawbacks, if it is possible?

If I can use a Crossfire-ready Sapphire HD3870 graphics card (“Two independent display controllers. Drive two displays simultaneously with independent resolutions, refresh rates, color controls and video overlays for each display.”) in an SLI-ready board to drive a single monitor, can it drive two monitors; same for an SLI card in a Crossfire-ready board?

Please note I havent ran either a crossfire or SLI system yet I will be soon hopefully, but these answers are based upon my understanding of the technology.

SLI and Crossfire is all about multiple graphics cards, one card even if it is crossfire or sli compatible is not an SLI or Crossfire setup until a second (or in some cases third or fourth) card is added. SLI and Crossfire are the way multiple cards are linked.

As far as I know you can run a single SLI card in a crossfire ready motherboard and a Crossfire card in a SLI ready motherboard, BUT you cant get two Crossfire cards to link up and increase performance on a SLI motherboard and you cant get two SLI cards to link up and increase performance on a Crossfire motherboard.

Im not quite sure what you are getting at here, if you are asking about a single graphics card with two DVI ports those two ports CAN be used to drive two different monitors at completely different settings, and they can be set up in many different ways, they can be used to try to look like one big screen or they can be used to create two desktops etc etc. When using a single card it SHOULDNT (I havent tested this to confirm) matter if its a sli card in a crossfire board or a crossfire card in a sli motherboard. I use this sort of functionality to drive a game on a monitor whilst simultaneously playing a movie on the projector for the kids.

It would be an odd decision to buy a crossfire motherboard and a sli ready card though if you are thinking of multiple cards for the future it would normally make more sense to buy one card already that you could just add a second one to later, as far as I know theres no reason you cant run a single SLI card ina crossfire motherboard and then at a later date buy two crossfire cards and put them in instead though.

Thanks to all for the help!