It doesn't have an onboard video output too? If it does you will have to remove the video card for that to work.
Would help if we knew make and model of computer. Try unplugging the video, hard drive, DVD Drive, PCI cards, anything plugged into USB other than KBM and minimal memory and start from the there and see what happens. If it boots to BIOS then start adding items until it stops again.
Other than RDRAM another computer which will not start without a pair of memory is the Apple G5 Power Mac, you even have to have them in the right slots.