I have given this advise on Hacker News before: on eBay 2-3 year old workstations can be had for 300-400 Euro. Typically these are workstations from companies that replace machines every 2 years or so.
These are typically equipped with Xeon CPUs, plenty of memory, sometimes ECC etc. Moreover, since they are usually HP/Dell workstations, they are certified to be compatible with Red Hat Enterprise Linux, so all the hardware is pretty much guaranteed to work.
This is very cool. I wonder how much the electricity costs would be if I let that machine stay on 7/24 for a year. Power, in Germany, is a bit expensive (learned the hard way).
These are typically equipped with Xeon CPUs, plenty of memory, sometimes ECC etc. Moreover, since they are usually HP/Dell workstations, they are certified to be compatible with Red Hat Enterprise Linux, so all the hardware is pretty much guaranteed to work.
Just to give one random example:
http://www.ebay.de/itm/DELL-Precision-T1650-CPU-Intel-Xeon-E...
Xeon CPU, 32 GB RAM, 500 GB SSD for 425 euro.