Mobo won't post with two 470gtx

Here's what I have currently:

Asus P5N-E nForce 650i with an e6750
4x 1G DDR2 800
2x 350G HDD, 1x 1TB HDD
800w psu
evga 470gtx 1280

I've been slowly building towards a new pc, picking up components when I have the money. I'm saving the new cpu/mobo/ram purchase (since they all have to be at the same time) until later this year, probably at the end of the summer.

However, I recently picked up a matching 470gtx since I'll ultimately want to run 2 in my next system, but I also thought that I could run them in my current system as well.

So, here's the problem...

I can insert either gpu by itself on my mobo and my pc will boot fine. However, when I flip the sli selector card to dual and plug them both in, my pc won't post, no display at all, and I get 1 long beep followed by 3 short beeps - which ASUS says indicates a video issue. I've tried unplugging everything but my main HDD and the 2 gpus, removing all but 1 ram stick, etc. but it just won't work with the 2 cards.

From looking around on google, I'm thinking it's one of two things. Either my psu isn't cutting it (I have 2 separate 12v rails for pci-e, one at 20A, one at 22A) or my mobo is having issues. Each 470gtx requires 38A, so I'm guessing I would probably be at the psu's limits, but I think it should at least post. I've also read many threads by people with the Asus P5N-E having trouble running a variety of different cards in SLI. It is an older board, and really not ideal for 2 of these giant cards.

I just wanted to get some opinions on whether it's the psu or the mobo or something else entirely, or if anyone has had any experience with this mobo running 2 newer power-hungry gpus.

Thanks!