Well, I had an almost identical problem to this at work today. It turned out to be a faulty CDRW. I don't want to assume, but you' have had your cd drive(s) unplugged through part of this process atleast? (disconnect floppy drive too)
Do the fans spin up ok when both ram sticks are out. (The fans should spin most times depending on motherboard).
Overall, everything attached to the motherboard fails. Harddrive failure, RAM failure, not booting sometimes, no beeps, no bios, you get the drill. That means it's one of four things; Motherboard, CPU, RAM, Power Supply.
Power supply: Have you tried a different power supply?
RAM: I'm not sure if you've tried other RAM (too tired to read the whole thread)
"oddly I thought I'd popped a ram chip so I pulled one out and it started up fine for a whole day. then same thing again. I started up another hd with windows mce and same thing."
Makes me doubt RAM is the problem. Perhaps the 'failing' power supply couldnt power two RAM sticks then eventually burnt out leading to your computer not booting at all
CPU: It's probable but unlikely that its the problem. Have you tried another one?
Motherboard: Replacing it is the fastest way to rule it out as the problem. But it's probably about the cost of a new power supply.
More things to try:
Check to see if theres any scorch marks around the power plug on the cable or motherboard.
Check for burst capacitors on the motherboard.
Check for excessive dust (the less dust the better. check in PCI and RAM slots too)
Do you notice any burning smells when its been on for sorta 3 minutes or so?