Sli

I have a Gigabyte MA790X-DS4, which I know does not support sli (although it does support crossfire X). But If I get the 9800gx2, which is two 8800gt's.. will It work, or do I absolutly need a sli motherboard. and not a crossfire one.

The 9800GX2 (and other dual gpu-single card boards) utilize internal bridges for SLI communication, in this case the BR04 bridge chip designed by nvidia. So basically the SLI communication is done through the BR04 bridge, your pci express card talks to the card, and the work load is split through the bridge chip into each of the two GPUs.

The BR04 bridge has 48x pci express 2.0 lanes, 16x lanes are reserved for the card as a whole to talk to your pci express x16 slot, and the remaining 32x lanes are split, so each gpu has 16x pci express 2.0 lanes to talk to each other. Data sent to the card via the first 16 lanes to the bridge chip, then that data is split between each gpu via its own 16 lanes.

So yea, it will work just fine. You can use a 9800GX2 on any motherboard supporting at least 1 pci express x16 slot.