I have 2 x PowerEdge 2800 Servers that are for sale. Each are out of warranty now though, and contain 2 x Single Core Xeon 2.8GHz Processors (64 Bit Capable), 7GB Ram, HDD Caddies to support 4 x SCSI Hot Swap Drives (sorry, will be sold with no drives!). They support ESX 3.5 fine (not sure about 4 though). Looking at £225 each.
Also can bundle in a Qlogic iSCSI HBA for each that I have for another £150 or so for each.