Comment HP Proliant MicroServer N40L (Score 4, Informative) 320
I don't work in a data center. But I think you might want to look at an HP Proliant MicroServer.
Basically it is an AMD laptop chipset on a tiny motherboard in a cunningly designed compact enclosure. The SATA drives go into carriers that are easily swapped (but not hot-swappable). It's quiet and power-efficient. It supports ECC memory (max 8GB) and supports virtualization.
Silent PC Review did a complete review of an older model (with a 1.3 GHz Turion instead of 1.5 GHz).
http://www.silentpcreview.com/HP_Proliant_MicroServer
SRP is $350, but Newegg has it for $320 (limit 5 per customer).
http://www.newegg.com/Product/Product.aspx?Item=N82E16859107052
Newegg also has 8GB of ECC RAM for about $55, so you can get one of these and max its RAM for under $400.
I just got one and haven't had time to really wring it out, but I did do the RAM upgrade. Despite the tiny enclosure, it wasn't too painful to work on it, and I was impressed by the design. The Turion dual-core processor has a passive heat sink on it, and the single large fan on the back pulls air through to cool everything. (There is also a tiny high-speed fan on the power supply.)
I'm going to use this as my personal mail server. It's cheap enough and small enough that I plan to have at least one put away as a hot spare; if the server dies, I'll power it down, move the hard drives to the spare, and I'll have the mail server back up within 5 minutes. Not bad for a cheap little box.