By default, I beleive it's set to allow windows to manage the amout you have... It's best to leave it like this for the most part because you don't want to set it to too little. Too much would just be a waste.
See, the problem with virtual memory is that it's so slow. RAM is MANY times faster than harddrives. Even setups like mine, there's no way for the harddrive to match the speed of the RAM.
So baslically, I think your system would adjust itself to as much as you need. I think most computers can make due with only 128M, but you will find some pretty crappy performace. I mean the less RAM you have, the harddrive has to work overtime, keeping up with loading things AND anything stored on it for RAM.
--edit--
I took it more as running more things. Heh, you will have less storage space if you take it up as virtual memory.