Old server was a 2.4GHz P4,
I'm very impressed with the new computer so far. Graphics are sweet! I used the intel DH57JG motherboard. Supports DVI/HDMI dual display, RAID. I used the Intel i6-661 cpu. 3.33GHz, dual core + hyperthreading, 4GB RAM, Dual 1TB HD's mirrored. I'm running Windows 7-64bit and run the webserver in VMware. Seems to be really quiet.
http://www.intel.com/products/desktop/m ... erview.htm

Here's the case I used
http://www.e-itx.com/gfi-g45.html