New build only POST's sometimes, usually doesn't - what's wrong

[5tephen]

I have an X-Pro 460W, Gigabyte K8NXP-9 nForce 4 board, AMD64 X2 3800+, 2x1Gb Corsair XMS3200 RAM, X1800XT and a couple of hard drives. All newly put together.
Everything worked fine for about a week, but now about 90% of the time the system will not display anything at bootup and judging by the lights on the CD drive, is in a continuous reboot. I have to power it on and off countless times before it switches on and I don't do anything extraordinary to get it working.

What could be wrong? It's surely not down to power supply is it?

 

[5tephen]

I tried everything I could including taking out the battery, running the minimum amount of devices plugged in, reseating & removing memory; nothing.
It's handy having a second machine to try stuff in. Turns out, it's what I was originally suspicious of; a faulty graphics card. Tried the X1800XT in my main machine which has an Enermax Liberty 500W which obviously should have no problem; the exact same issue as the other machine. Off to OCUK we go