actually, i am pretty sure you CAN. The problem is, the third card won't be that dramatic of a difference. Knowing him though, if it can be done, he will want it done. I'm almost certain you can do it. If it doesn't work, i will drop back to 2 but right now its looking like 3.
edit: just checked and it might not even be physically possible...not enough SLI jumper slots. hmmmm...dammit. lol