It has always seemed odd to me for people to be unsure of why they did things.
Mac OS X has been pretty late to the SSD game, but even as far back as 10.4, which was well before SSDs were even a thing on desktop level computers, if you had a good SSD you should've noticed a pretty epic performance boost. I run them in my portables for durability reasons, and I'm getting to the point where I'm going to start putting them in all of my desktops for boot+apps. Maybe the reason you got rid of it was because your OS + apps exceeded the capacity of your SSD?
The 1100 & Co are an interesting bunch, although if I was going to put it in periods, I'd say they're second period/generation machines, purely because they're "not the 320/540" -- and therefore, normal x86 computers, as you indicated.
Keep us posted on what you do with them though, be it transplant more modern components in there, or run linux on the PIII guts.
As a sidenote on power consumption -- servers don't always draw what their PSUs are rated for. If your Woodcrest system has iLO or DRAC/iDRAC or anything similar, you may be able to get a real, current, power usage number therein. My Westmere-EP system (one socket populated) with eight disks pulls 160W when running with the CPU pegged, and around 120 when "idling" (as idle as the machine in question can be.) And this is on a PSU rated for approximately 560W. (I've considered grabbing a second PSU for it, that would be for failover operation though, rather than supplying more power. There are also 800W PSUs available for my system, and the next generation forward has dual 1100W PSUs available for it, but that's for configs with things like four big workstation GPUs.)