If you have not fixed the size before, then the PageFile will have "chosen" its own size already. I normally look to see what it is currently, add about another 1GB for headroom and fix at that. You can reduce the headroom if you have a smallish disk but the current value will be a reasonable approximation to the most you use so you probably want a little extra above the current size.
The key thing is to set the min and max the same before all the defragging - a fragmented PageFile is a complete performance killer on Windows.
BTW, unless your computer is of the "latest and greatest" variant, eBay is often a good source of cheap "upgrade" memory. WinXP can handle up to 4GB (if you have the slots to put it into) and even going from 1GB to 2GB will make quite a difference (after defragging too).
Paul DS.