Salut Solidworks better with vista or windows XP 32 bit or 64bit
64-bit by far. With a 32-bit OS, you can only use 4 gb of RAM. 1/2 of that going to the OS and the other 1/2 going to any applications. Starting with SolidWorks 2008 SP3.1, Vista 64-bit is approved. I've run into little issues with SolidWorks 2009 x64 and Vista Ultimate x64.
I think VISTA 64 bit is being used quite a bit now with SW. Just make sure that you have the drivers you need. TOP engtran.com
one important point for your files, keep all your work on a separate hdd; vista has a habit of locking up failing to boot requiring complete reinstallation- it's bad enough when your os crashes, keep your work modular and spare yourself a lot of pain.