From my experience<*pops*> are often the power supply giving up the ghost; there's not a lot on the motherboard that could make that kind of noise. Problem is, when power supplies go, they sometimes take things that are plugged into them with them.
I agree with Greg that, if you can find a replacement power supply, you should try that first, as it will be the least expensive part, and may do the trick. If the motherboard was damaged you'd probably see evidence of it - burn marks around connection points, etc.
If the HDs are fit, you could also try moving them to a working PC with the same ports (e.g. SCSI, IDE, etc) or an external kit. You probably couldn't boot from them, and you may have to play with jumpers to make them recognized, but you may be able to read from them; at least, if you're familiar with how a healthy hard drive sounds, just connecting them to a power source might tell you whether they were blown up or not. As Gail mentioned, if they were RAID drives, this becomes much, much more complex (and your best hope lies with the issue being just the power supply).
I'm sure you'd have noticed if this was the case, but if you were using a CRT monitor (you did say it was old), they have tubes that can make <*pops*> when they go; in which case you'd have no video, but would still have power to the PC (again, doubt you would have missed this, just trying to be thorough).
I haven't been exposed to it much lately, I used to do a bit of h/w support, so if I can help, just let me know, Lynn.