I'm a month late. But if you can, I suggest measuring the voltage rails with a multimeter while it's powered up. Maybe it's just not getting acceptable voltage. Put the negative on a safe ground (like the PSU case), and carefully backprobe the positive into the ATX connector. The plastic shrouding of the connector makes it pretty safe, it would be hard to short anything.
Also check the Intel manual for their procedures on doing a full reset of the CMOS. Those manuals are insanely detailed, it's something I love about Intel boards but they also are pretty strict. If one thing isn't exactly to spec, they will refuse to run. They're the polar opposite of an enthusiast overclocking board, which will try to play along with anything.
On that point, I think it's possible that a terminator is required to run a single CPU (not recommended, but mandatory), but I'm not sure on that model.
I can't quite read the ratings, but modern power supplies tend to be relatively weak on the 3.3v+5v rails which older motherboards draw most of their consumption from. So although that PSU is rated 500W total, it might not match up well with that board. I'm just speaking generally, I don't have any experience with that particular PSU or motherboard.
I had a situation once where an older motherboard gradually killed the 3.3v rail on a newer 550W PSU. It took about 1 or 2 weeks for that to happen. It turned out the rails that actually mattered were weaker than my 300W PSU, which went on to run that system perfectly.
There's a beep code for CPU failure? That's weird - normally the CPU has to be running for the BIOS to do anything at all, so the failure must not have been absolute. Unless that board has some other processor on board for doing those type of checks.
The SDS2 is pretty much the ultimate P3 server board. I remember looking for one of those about a year ago and giving up. That price was a steal! My guess would be that it was stocked somewhere as a backup for other servers, and never needed to be deployed. Obsolete server gear has a way of hitting the 2nd hand market very cheap sometimes - cheaper than the consumer equivalent, very often.
I'm a bit superstitious about powering up something that's been asleep for that long. I think it might be better to wake them with a minimal configuration (single low end CPU, little RAM) and just let it sit in the BIOS for a while, before ramping up the load with added hardware. I don't know if it really makes any difference though.