You can boot without any HDDs, and you'll still get a picture. It's not that.

Is there any beeping or anything? If so, it's most likely your RAM.

If there's no beeping, I say that it's your bios. You can try resetting your bios if you have the manual (or can look it up online). It'll just take setting a jumper and then giving it power. That should default it back to onboard display. Otherwise, try onboard video, an agp video card, and a pci video card (and pci-e if you have it). if any of them works, then it's definately your bios.

Try and report back.

What is it they say about silence being golden?