Note what was said above about "server" vs. "workstation".
There is a difference, and a big one.
I've been buying off-lease IBM dual-socket workstations since about 2001, but only the workstations (IntelliStation M-Pro and Z-Pro) - NOT servers.
Those workstations are set up to actually run desktop apps and utilize video cards, albeit in a heavy-handed fashion.
Rackmount servers aren't so friendly to doing anything other than acting as remote servers. They'll run a VM for you, and obviously play fileserver.
They won't be your first choice for running Borderlands 2 or Adobe Creative Suite 6.
There's also a noise and wattmeter penalty for those 8 cores and big power supply.
I continue to dabble in the heavy iron - my latest IBM toy is an IntelliStation Z-Pro 9228 workstation.
It has 2 each Xeon X5365 quad-core 3.0Ghz processors, and 16Gb of FB-DIMM memory. It's also hopelessly obsolete in 2012, IBM stopped selling them in 2009.
IBM, HP, and Dell professional-grade workstations are built like the proverbial brick outhouses, but for entirely different purposes than typical gaming desktops.
For reliability, they have long self-testing Power-On Self Test routines, use Fully Buffered DIMM memory, and run big noisy fans with huge heatsinks, SAS/SATA RAID controllers, and chassis intrusion monitoring hardware, amongst other things.
The money I saved on buying my off-lease IBM (as found on eBay or other websites) went into a Koolance water-cooling system to kill the fan noise, a Radeon HD6870 for gaming and Creative Suite 6, a 256Gb SSD primary drive for sheer speed of file access, and stuffing those 8 sticks of expensive and obsolete FB-DIMM memory into the slots. I run Windows 7 Ultimate 64-bit, and the computer reports a Windows Experience Identifier score of 7.8. That's the good part.
The bad part is that one could get a current quad-core or hex-core i7 system that would also do the same performance-wise, with less wattage draw, footprint, weight, and noise.
That having been said, if you just want to play with industrial-grade hardware, it is cheap when sold as surplus. I used to buy at
www.pcsurplusonline.com for a long time, but they've somewhat dried up in the workstation and server department. Sometimes
www.geeks.com gets corporate machines in, as do a few other sites out there.
I ran SETI Optimized for a few weeks earlier this summer. I averaged just over 10K credits/day, but the heat in the office and the spinning wattmeter shut me down in short order.
My next machine will be the Lenovo ThinkStation D30, with 16 Xeon Sandy Bridge cores vs. the 8 relic Clovertown cores I'm currently using. And yes, I'll test it out on SETI.