My main machine running TWS is a couple-year old Sony Vaio 2.4 ghz desktop, with 1.2 GB RAM and a Matrox dual monitor card and two Sony 18.5 LCDs.
A very solid machine IMHO, but probably over-priced. I've never had any problems with it, but I don't think I would get another one given a cheaper choice.
As for other machines you might look at, my daughter has a newer Gateway that works great, and we use many many Dells at work. About the only problems we see with the Dells, are occasional disk drive failures. Don't know if that is a common problem with them or not. Would certainly suck to have that happen to your trading machine.
Maybe somebody should do a poll on the Hardware thread and ask what Machine they use and what Op system, and post what kinds of problems they have had?
I've been thinking about getting a new system too...