My current favorite is the Altus 600 from Penguin Computing. Cheap chassis with dual sockets and 16 RAM slots. More slots means cheaper chips. 2GB chips for that board are about 300 bucks. Throw two 2.2GHz Opterons and 2x 300GB drives in there. Add an SATA controller card, 4 SATA/CF adapters and CF cards (in the CD-ROM bay), and you've got a nice machine: 1U, dual processors, 32GB of RAM, 600GB of platter, 32GB of flash for about USD$7500.

Expensive, yeah. 5 grand of that is RAM. You could get away with 4GB of RAM and still have a fucking fast web/app server as long as your active dataset fits on the flash.