You asked about using a PSU from another model. I believe the GX280 uses a 24 pin PSU when many of the earlier Dell Optiplex/Dimension/Precision models use a similar PSU but a 20 pin connector (more readily available). I'm pretty sure it uses the same PSU as the Dimension 8400 and probably lots of other Dell models. The GX270 (and all the Dimension and Precision equivalents) use 20 pin connectors. That being said, you can score 20->24 pin adapters pretty cheap (I'm actually using a 24->20 adapter on my GX270 in order to use an aftermarket 470 watt PSU).
A PSU would be my next step if I was troubleshooting.
Good luck and let us know what you find. Keep in mind that you will experience fit issues with many aftermarket PSUs. Search around the site to see what works in the aftermarket. Dell loves their proprietary parts.
I stand corrected on the PSU. I have a GX280 at work and never counted! It shares so many similarities with the 8400. I think the 8400 has a 24 pin. Re: CPUs, I've seen socket 775 CPUs go fairly cheap on eBay, but if that doesn't work, then what?
I'm watching this thread out of interest for my PC at work. If you get a new CPU, just don't get a 570J or 670 (both 3.8 Ghz). I can't get either to work with the GS280.:smileymad: