i stick with what works for now which are: OSX on my G5 and Windows XP on my PC. The thing with XP, even the 64 bit version, the drivers are still shoddy from 64 bit stuff being so new. I would recommend keeping things simple, investing in good gear to record and focus your efforts on recording as well as you can with the existing OS on your computer.
Kit, Nuendo has been reported to work well with Vista but as others in the Steinberg boards are quick to report, the amount of bugs with Vista is too much liability to even consider a lot of core users to jump ship from XP just yet.