Here's a thread you might find interesting. It discusses buying a secondhand server with two L5639 processors. The long and short of it is that I worked out you can get around 12 points per watt with one of those servers. Probably better can be achieved by reboxing the unit so you can use larger heatsinks and fewer slower fans.

For a UK buyer, shopping round ebay europe rather than worldwide is probably best as you can avoid the import duties.