Need help buying powerful pcs for large data analysis

Lots of reviews around the web on desktops in general but I can’t find much to help me with High-end Desktops (and I mean Desktops designed to handle really large dataset analysis – think scientific applications.) Specifically, in this case, people working on analyzing the human genome.

Lots of options out there – all of the major Desktop vendors have products in this space but I’m wondering if anyone has experience with these and can suggest vendors to purchase or avoid.

The system must run Windows XP and will be running SAS, S-Plus, R, etc. Currently there is no 64-bit SAS license for windows for this project. While performance is primary consideration of course cost is a consideration too.