I've used Ibm, HP, Gateway, acer, and Dell. Avoid the Dell, it's the only one I've had problems with. I'd go with Rob's advice (outside of buying a Dell ), just buy as much as you can for $500 - $600 and get lots of memory.
Side note: I've actually been playing with some vmware (VirtualBox actually) running xp, linux, and even mac within my vista client. Internet, Network, and all ports (including usb) are available to the OS running in the virtual system. Might be an option for your need for windoze if you do go with mac since they have a download to run VirtualBox on mac. I've never had an actual mac so couldn't tell you what it does out of the box.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault