For cheap, where power consumption isn't an issue, I've used these : IBM Thinkcentre
With 2GB of memory it runs XIPAR with no memory swapping.
I'm building up another node running ACID, with no problems so far.
In addition I have a flock of different scripts running at different times, again no swapping.

And my experience with this particular EBAY seller has been great.
No financial interest on my part, just a happy customer.