K8NXP-SLI video card question

Question about this board. I recently purchased this board and installed
it in a case with a 300w power supply. After installing everything I
tried to boot and the system would not start up. Eventually I figured
out that the problem was the Gigabyte video card I had installed based
on a nvidia chipset. I removed the video card and installed an old PCI
voodoo3 board and the system started. I sent the card back for an RMA
and I ordered an ATI sapphire based card. I received this ATI based card
yesterday and installed it but again the system wouldn't boot. So
evantually I decided to play with the normal/sli mode module. When I
switch the module to sli mode the system boots, if I change it back to
normal then it doesn't work. I have the card installed in the PCIE slot
nearest the CPU, when I tried the other slot the machine would only give
beep codes indicating that no video card was installed. When I first had
the problem I also tried using a bigger 450w PC Power and Cooling power
supply but still had problems. Any info on why the machine wouldn't boot
in normal mode?

TIA