Currently using a Dell C1100 (cs24-TY) with 2x quad core L5520 CPUs and 72 GB of RAM. Ebay has them plenty for anywhere from $500-$699 with out HDDs, trays included. Unfortunately this server does not come with a RAID controller, and is quite loud when running under a load.
This particular server does not come with hardware RAID, and VMware does not support software RAID. It means you will need/have to use individual drives and wont be able to take advantage of any kind RAID setups for a speed/IOps boost.
I bought it at the moment due to its cost, size (1U), performance/RAM (72GB), and requirements I had in place, originally.
I have populated it with 2 x SAMSUNG 840 240GB and 1 SAMSUNG 850 EVO 250GB SSD drives, and a 2 TB WD 7200 RPM drive, which is used primarily for backups and VMs that require low IOPS.
I am currently running several nested ESXi 5.1, and 5.5 on it, and its handling it all quite well.