Which is the best operating system for CAD/CAM?
Sometimes you can't choose. However, these days your best choice is Windows 7 x64. If you make any serious hardware change XP will need to be reinstalled and you might end up with a complete reinstall of everything, Win7 handles this fine like a Frankenstein, just plug the power and it's back, alive and well :)
I think the question should be more like " the hardware..." For example Dassault has lists of certificated workstations. (also UNIX)
If you are looking for more performance my adwise is get a good graficcard like nvidia quadro
Any 64 bit operating system would be best as it manages memory better than a 32 bit system. 7 is a newer operating system and is the best choice for an operating system. Win 7 would make use of newer features that are/could be implemented in a software to further speed up your computer. However if you have a strong computer it won't matter what you have as an operating system. Are you looking to buy a new computer or upgrading an old computer?
(Simple answer is win 7-64 bit would be the best)