Yes you can use either 2 9800GX2's or 2 7950GX2s but first of all IMO there are better options that them,you can get single cards with better performance.
Also the difference between dual 8x and dual 16x isn't much but since its Quad-SLI then you may see a little performance drop (not much though) compared to dual 16x on high resolutions.
thank you for responding, on second thought how do you think the new (semi new) nvidia gtx295 i believe it was, perform on this board. i think they were the recent cards to support quad sli and one of 3 series of cards to support quad sli, using 2 dual gpu cards that is.
do you think the 570 chipset will bottleneck and performance?