I was a little uncertain about it so i did a search and it seems that if you put the cards in the first and the third slot,then the third slot will work @ 8x,whereas the first one will work fine on 16xhttp://hardforum.com/showthread.php?t=1474341
"If the third slot is unoccupied, the first two will both run at 16x. If the third slot is occupied (even if the second isn't), they will run at 16x/8x/8x."
As for the sound card,well there are many good PCI models available, but since you already have ordered one, then you may want to keep it.
So just to be sure that everything works fine at their correct speed,put your VGA cards in the first 2 PCI-E slot and the sound card in the last slot