The dell dimension 4600 motherboard is not posting. I know it's got some life in it because when I change out some of the PCI cards with non working ones it doesn't power on. Additionally, the processor fan does not spin so I think this is the number one source for no post at power on.
Sounds like a RAM or PSU issue. If its RAM, chuck the PC out, no luck in finding DDR(400MHz) RAM. If its the PSU, I guess it could be worth it.