It could be as simple as an old CMOS battery. Did you try that?
Google "The Dead PC" for how we work a dead PC. Also I would not get a motherboard. Those old dimensions seem to be out there for 99 or less bucks for a working unit. Be sure to check out Groupon.com for such as well.
I have a Dell PC E520 Dimension.
When I press the start button it lights, but the monitor receives no signal. Nothing is happening. I think that the MOBO is blown. I think the best solution is to buy a used tower off e-bay. Do you agree