I'm still on the "Old Reliable" desktop most of the time. DUAL P3-933s, Gig of RAM, 3 - 60GB drives, with a good 256MB video card, and Windows 200 Pro. This machine is at 5 years old, and still has plenty of life left in it. I do graphics and engineering math models on this box all the time, and it can easily keep up with any P4 box I've ever used.
Then I have "THOR" for serious processing. A Quad-Zeon P3-650, Compaq Proliant 5500. 2GB ECC RAM, SmartArray 3200 controller, 9 - 18GB SCSI 3 drives, and redundant power supplies, running FreeBSD 5. "THOR" was bought off eBay for only $500, it was immaculate without even a spec of dust in it. It was about $100 for the shipping because the size and weight (over 160 pounds) and they had to ship it on a pallette. Not your average "Home PC".
Olde Computers ROCK!!!
The only modern computer I have is a Sony Vaio P4-2.8 Laptop.