I think in nuendo you can select where your temp files go.
However why not get two fast drives. I don't think you should try and save a few nickles on one of the most vital parts of your box.
Windows' overall performance will be much better on a fast system disc.
Also think about having REALLY silent cpu fans and powersupply (
http://www.zalmanusa.com/) in there and a vid card that has no fan at all. Some pc's sound like a bloody vacuumcleaner.