I'm running XP on a 1.2 GHz, or at least I will be soon. I'm actually got a beta of the Win 2003 servers, which expires in November, it's basically on there just because I wanted to play around with it, but I have a felling I'll be switching back to XP here soon. Obviously tweaking something in ther render setting could have a big influence on my render time changing, but I'm talking about little changes, like moving a hand over a couple inches because I don't like the way it looks. Usually what I do is just use the draft mode until I get things where I like them, save it, restart my computer, and then do a full blown production style render.