The Mac Pro doesn't support SLI technology, so those 7300GTs would be working separately, meaning basically you will only get the total performance of a single 7300GT.
I may be wrong, but I think the fastest add-in PCI-E nVidia card available for Mac is the 6800GT, but I don't know if that one has been tried in the Mac Pro yet. I know some of the iMacs have GeForce 7600GT cards in them, so someone may be able to modify that BIOS to flash on a standard PCI-E 7600GT card, or this may have already been done, who knows?