Does your motherboard beep for no VGA when you turn it on or is there just no image?
I am starting to think my problem may be I/O based and the problems almost always occur during CDROM access (I have tried 3 different drives and both IDE channels.)
Initially it didn't (was able to boot up and shutdown using the power switch, just no video output), but after the third attempt it beeped a few times and the onboard graphics took over instead.
If you've swapped a few drives thus far (and presumably ide cables also) then it's looking like a suspect motherboard if the ram turns out ok I guess. Perhaps you'll get to recap something yourself too at this rate!