I'm getting ready to purchase a new computer soley to run CFD. Should I get Windows 2000, NT, Linux, or something else?

I would recommend Linux. Fluent runs very well on Linux and the port is reasonably bug-free. What is best for you probably depends on which operating system you feel most comfortable working with though. Fluent runs well on NT also, although I've been told they've had more challenging bugs to fix on the NT version (have not used it much myself though).

(1). Most of commercial CFD codes were written for mainframe computers or workstations, therefore, the commonly used operating system is UNIX. (2). But you need to remember that implementation of UNIX is machine dependent. But you should have no problem in using UNIX workstations, whether it is HP, SUN or SGI, because these have been around for a long time. (3). Most modern commercial codes have included some graphic programs to process grid and results. And the computer graphics are very sensitive to the hardware of the computer, in addition to the software part. (4). So, when moving to the PC platform, it is important to know whether you are going to run the code exactly the same way as the workstation environment. In most cases, this is not possible because of the graphics hardware and software. (5). Since most of the PC versions are originally developed for the workstation environment, you will have to check it out first. (6). In other words, the hardware you are going to use also will have impact on the operation of the code. (this is especially the case for the PC hardware)

We run CFD software on NT and Linux and we have found that Fluent is faster on Linux (same computers).

We found the opposite. Fluent on NT is faster. Beside this there seem to be problems with some linux-versions.