Computer Powers On, but won't boot up

I have an older computer that I recently decided to upgrade slightly. It's an A-Bit KN8 SLi ready motherboard. I had two nVidia 7300GTs in it, and when both of those cards are in, the computer will boot up fine.

However, if I remove one of the two 7300s, or try to put my new video card in (GTS250), the computer will power on, but never boot. The CD/DVD rom simply blinks on and cycles like it normally would on start-up, but it does it constantly, as if the computer is stuck in a constant power-up.

Somewhat lost on why this is the case, as I've snooped around the BIOS and saw nothing that directly related to SLi, or why it wouldn't boot with both cards in. I really am stumped!
Help please.