Another vote here for Mac, but get one that can run Parallels (e.g with the Intel processor) so that if you want to you can run Windows. Buy XP if you can, of course.
The reason I recommend this is that there are a lot of obscure bits of add-on hardware, and at least one major area area of software, that aren't very well supported by macs. For example, there is only one USB digital TV receiver for Macs and it's pretty expensive, there are at least a dozen for PCs. Most USB microscope cameras and web cameras, and many scanners, are not supported by Macs. And there is now no route planning / satnav software that runs under OS-X other than a program for yachting - Google Maps will allegedly eventually add GPS support in all versions some time, but it hasn't happened yet and you need a network connection to use it anyway. All of those should be soluble by running Windows under Parallels.
