Machine won't boot with 2 video cards

[hyzerfool]

I'm trying to add a second video card so I can go from 2 monitors to 3. I only have PCI or PCIe x1 slots available.

Existing video card is nvidia 7300 GS (PCIe x16)
New card is Sparkle nvidia GeForce 8400 GS (PCI)

Machine boots fine with either one in by itself.
When I try to boot with both cards in, I get message telling me that no boot device was found.

Motherboard is Intel DP35DP w/ 8 GB of RAM
Power supply is Antec BP550 plus

OS is Windows 7 64-bit, but that shouldn't matter right now as I can't get to the OS...

My first thought was a power supply issue. The machine has 3 hard drives (2 in RAID array), I disabled the 3rd hard drive and the DVD drive (to reduce power load) but got the same result.

I haven't found anything in BIOS that looks particularly promising.

I'm at a loss. Any suggestions?

 

[juicycrapachino]

Did you connect the SLI bridge properly? Double check that and also it might be the slots of each card. One is PCIe x16 and other is PCI, that might be the issue...But I'm not %100 sure...

 

[Rustyy117]

Did you connect the SLI bridge properly? Double check that and also it might be the slots of each card. One is PCIe x16 and other is PCI, that might be the issue...But I'm not %100 sure...

You cant run a 7300 GS and a 8400 GS in SLi

 

[hyzerfool]

Yes, not trying to do SLI. I believe SLI uses 2 cards to drive 1 monitor. I want to drive 3 monitors.