Dual Video card issue

Here's a weird one.

I have an Asus Mn2-e Sli Mobo, with an asus 8800 gts 640mb video card. I added a 2nd video card(GT6800) and it wouldn't boot at all.
I tried a different one (Geforce FX5500) to run my third monitor and it booted fine. I installed vista to another drive, and when I boot I don't even get past the ram check, it just hangs there.

Removing the 5500 and it's fine again.

Any ideas on what I could do, I'm really missing my 3rd screen, I'm cheap so I've got 3 small screens:)
My system specs are

amd 4400+ X2 64bit processor, asus m2n-e Sli motherboard, asus 8800 GTS video card and a Geforce FX5500 to run my third monitor
booting ubuntu xp and vista on different drives.

In order to run two video cards in SLI the cards must be identical and are then connected to each other via the SLI bridge so what you are trying to do with two different cards will not work. The way around this if you really 'need' a third monitor is to install a PCI graphics card separately to your PCIe and run the third monitor from this or get another ASUS 8800GTS and run it in SLI mode.

That's what I'm trying to do, I'm not attempting to run them in SLi.
I did install the second card separately, and the two of them worked fine together until I installed vista.
now I can only run one.