What Operating Systems best supports CFD? And what are the hardware requirements For CFD?
You have options all the way around. There are CFD suites for Windows as well as the 'nix variants.
My personal preference is for 'nix based solutions and the CaeLinux distribution (www.caelinux.com) has a live disc image from which you can run free/open source tools for FEA/CFD..etc. Or you can go in the other direction and spend many thousands of $$ on some of the Windows based variants.
As far as hardware goes, generally more is better. The workstation I'm doing my FEA and CFD work on has 2x 8core Opterons and 16GB ram. Works flawlessly for what I'm doing at the moment.
It's always going to come back around to the old saying 'Speed costs money, how fast can you afford to go?'