To determine your memory requirements give the PC a workout and check commit charge peak in task manager. That will tell you the peak memory usage of the programs you're running.
RAM+PF must exceed commit charge peak or you will generate an out of memory error. That's how much you "need", so conversly, commit charge peak-RAM=PF *requirements*
Sizing it will not necessarly yield performance benefits. However *if* the system has to manage the size a lot you can end up with a fragemented PF. That will cause system slowness. The answer to that is to either set the PF min=max where the min. is significantly higher than commit charge peak (so you never run out of memory) or let the system manage it, and use a defragmenting program that will defrag the page file.
Real world, with 2GB (edit: running standard "desktop" apps or games) you will not be paging much at all. Because of that, the PF will never need to grow, certianly not on a regular basis that would cause fragments in the PF.
Leave it to system managed, and use a defragmenter that will defrag the PF if it does happen to get fragmented.