Having too much virtual memory will slow down your system to little it'll crap out. The general rule of thumb is 1.5 - 2 times your physical ram. However generally I've found it's best to just let windows manage it for you. In some environments where you are only running certain applications specifically you can get a slight boost by setting the VM yourself. However on a typical home machine it can actually cause problems as windows really hates to be told what to do.
I have tried changing mine around and kryyst is right. It's best to let windows manage it for you unless you have some extreme graphics rendering. Or a program that really needs the extra VM. That said, Here's some info on Virtual Memory which is called pagefile (in Windows NT versions including Windows 2000 and XP: pagefile.sys). Other OS's (Windows 95/98/ME: Win386.swp) it's called Swap File.
Bill James MS MVP has a convenient tool, ‘WinXP-2K_Pagefile’, for monitoring the actual usage of the Page file, which can be downloaded here. A compiled Visual Basic version is available from Doug Knox's site which may be more convenient for some users. The value seen for ‘Peak Usage’ over several days makes a good guide for setting the Initial size economically. Hope this is of good use and interest to you.
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Bill James MS MVP has a convenient tool, ‘WinXP-2K_Pagefile’, for monitoring the actual usage of the Page file, which can be downloaded here. A compiled Visual Basic version is available from Doug Knox's site which may be more convenient for some users. The value seen for ‘Peak Usage’ over several days makes a good guide for setting the Initial size economically. Hope this is of good use and interest to you.