Here's a quick video to show how to change the Virtual Paging File size after upgrading RAM.
The video is for Windows XP.
Right click on My Computer-->Properties-->Advanced-->first group of settings-->Settings-->Advanced (again): Virtual Memory
One of the days I hope to get around to posting same for Vista.
Rule of thumb:
Minimum cache size should be about 150% of actual RAM. For maximum, you can pretty much double the minimum.