Dual graphic cards

I was wondering if i get dual graphic cards does it matter if they are they same? I have http://www.bfgtech.com/7900GTX_512_PCIX.html and if I had two of them in my comp I doubt it would fit

Yes.

Currently at least, SLI requires that both cards are identical in terms of GPU and clock speed, although the level of firmware or even the manufacturer of each card is no longer critical. With the right tweaks you can even get away with having different amounts of memory, but both cards will then only use the same amount as the smaller card.

The same isn't true for ATI's Crossfire. One card has to be a crossfire card, the other just a standard card, and they don't even have to have the same level of performance, although I believe they have to belong to the same class (eg both X800s). However, each card will still have to do half the work, so total performance will be dependant on that of the slower of the two cards.

Regarding that particular card, it is SLI capable, so if the same is true of your motherboard then it must be possible to fit two cards otherwise either the cards or the motherboard could not be defined as SLI capable. You wouldn't necessarily have to get another BFG card, but it would still need to be a 7900GTX and all the versions I've seen are more or less the same size. Whether you have room in your case, or for other cards, is a matter only you can decide though.

is your MB support SLI?if its yes with buying second card your performance increser about 40-50 percent in some game.

If you are going to use SLI they must be the same, but if you are just gonna use one card for one monitor, the other card for another monitor, it doesn't matter much at all. i have a computer with a agp ati card and a pci nvidia card and it runs the seperate cards fine (graphic drivers were a b**ch so I recommend sticking with one vendor)