for Nvidia's SLI
all you will need is two cards that are, obviously, SLI capable. it will say this on the box, and you can always tell if they have a small connector along the top edge of the cards. so if you have that connector it should be SLI-able
i think all of Nvidia's PCI-E range from the 6600 up are SLI capable.
you can mix n match vendors, but the cards must be of the same type. for instance you can match a XFX 7900GT with a BFG 7900GT. the clocks should match to that of the lowest clocked card i think, but i would suggest plumping for same make also.
again you will need an SLI capable motherboard, that will have 2 x 16xPCI-E slots. the minimum is for these to supply x8 speeds to each slot when in SLI mode. alot of High end motherboards now supply full x16 speed to each slot when in SLI mode.
some one will have to check me on this, but i think Nvidias new 8800GTX actually requires that your motherboard supplies the full x16 speed to each 16xPCI-E slot.
for ATi's crossfire its abit different. i suggest you skip completely over the X850 crossfire due to its limitations (1600x1200 @ 60hz limit)
for X850, X1800, X1900, X1950XTX Crossfire you must have purchased a special "master card" , the master card has on it a special compositing chip to combine the outputs of 2 graphics cards. these cards are usually labled "crossfire editions"
once you have your master card, any other card of the same type will do. (ie X1800XT + X1800XT) though i think it is possible to match say a X1800XTX with a X1800XT crossfire edition, the clock speeds will just match to that of the lowest card.
with Ati's brand new X1950Pro, they have now moved the compositing engine/chip onto the actual GPU die itself, and the X1950pro's now sport similar connectors to Nvidias SLI bridge connector along the top edge of the card.
so in summary
with nvidia you may, mix n match vendors and so long as you have an SLI motherboard and the two cards are of the same spec (ie 7900 + 7900) and can supply the power it should work
also Nvidia drivers have alot of game profiles so that you dont need to select which rendering mode to use to get the best performance (though i found AFR2 to be good for most)
with ATi X850, X1800, X1900, X1950XTX require you to purchase a "crossfire editon" card, and then pair it with any normal radeon of the same type. (ie X1950XTX CF + X1950XTX)
remember that the X1950pro is the exception in that it has the compositing chip built into the GPU and makes use of SLI type bridge connections along the boards top edge. in this case all you need do is buy 2 X1950pros
do you need any help understanding how multi-GPU rendering works aswel?
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