I am looking at BUILDING a new computer using the I7 Intel CPU. All the boards that I have seen support cross fire. My question is can you use any two of the same video cards or do they have to say that they support cross fire? It seems that most ATI cards support cross fire. Nvidia has no mention of it at any price. Thank you much for your help.Crossfire is an AMD/ATI technology, lets you use more than one AMD/ATI card together. Nvidia's equivalent technology is called SLI. You cannot use an ATI and Nvidia card together in Crossfire or SLI, only cards form the same manufacturer. Nvidia cards MUST be the same model, I'm not sure about ATI cards but I think they're more flexible - do check on that. Performance will be limited by the slowest card in either case.

Hope this helps.Thank you for your reply. I don't understand how 2 cards work. I mean you only hook your monitor to one? How will two cards help?The two cards are connected together internally with a bridge (at least, with SLI they are, and I'm fairly sure Crossfire is but not 100% sure off the top of my HEAD). They then share the processing between them, and output to the monitor through whichever card is connected to it.
The rendering methods are different, but they can share the work by rendering alternate frames, different parts of each frame, etc.
The performance boost isn't 100% as there are OVERHEADS and restrictions, but in some games it can come close to that.

Hope this helps.Yes, that was very helpfull. I was just looking at a board that will hold 3 cards. That seems a BIT much to me. I really thought that two cards was a bit much. I mean if one card will work to play a game then why would you need two. Then you just explained it to me. Very good thank you for your reply and time.for the one with three... I'm not even sure if SLI and crossfire supprot that...

but if they do three would have the same benefit, (although less pronounced).

Of course you don't ALWAYS need to put a video card in the slot.3 way SLI and Crossfire, and even higher, are supported and available, but not on all cards and boards/chipsets.
Generally multi-card solutions don't scale very well past 2 cards, as not all games even scale or support 2 cards.
As BC rightly says, just because the board and cards have the capability doesn't mean you have to use it, it's just another upgrade option should you want it.To add, Crossfire does work with two different cards. However, all cards involved in the crossfire setup clock down to the slowest card's specs.

With that in mind, I'm not sure anyone would want to crossfire 2-3 different model cards...