Thanks all for the advice - I'm pretty good with software, but not much of a hardware person (I initially bought this as bare-bones case, board, processor and memory, added some RAM and graphics card, HD and a few extras, and that's about the extent of my PC hardware knowledge)
I just disassembled the entire thing (took a while to figure out how to dismount the CPU fan and heatsink) and cleaned the dust off of everything, checked pins and firmly reconnected all parts.
As for hard drive failure - no, I haven't ruled it out as a possibility, but it's not what is causing the current difficulty:
When I push the power switch on the case (front panel) it causes the DVD to power-up and the CPU fan comes on, so I'm assuming power supply okay (however, coudl it be that the power supply is PARTIALLY damaged but not providing sufficient power for boot?) That is all that happens. Completely blank screen, no beep, no memory scan, no diagnostics, no BIOS, no error message, nothing. It's frustrating as hell, because everything LOOKS okay and is firmly in place and properly connected. I just tried booting with the absolute minimum (disconnected DVD drive, HDs, and all expansion cards) and same result, so it's something with the power supply, processor, memory stick (is that likely?) or mobo.
Looks like I'll have to do some swapping, if I can find somebody with a P4 processor and/or P4 mobo willing to let me experiment...
I will NOT be taking it to a repair shop, seeing as they will charge $100+ and then more for marked-up parts. At the current prices, if it comes to the worst I will just replace/upgrade parts one by one (starting with my best guess, which would be the motherboard) Worst-case scenario is that I have a decent brand-new system for about the same price it would have cost me to have some repair techs just replace the motherboard. Now I'm just hoping my data isn't wiped from the HDs. :(