Microsoft recommends a max of about 1.5x your system memory, in this case about 3000MB, however you can technically set it as large as the free space on your hard drive.
Understand though that you dont want it storing too much on virtual memory so set it to be fairly small(512MBish), this forces it to keep more stuff on the RAM which is far faster than the page files stored on your HDD.
Technically speaking, the "Virtual Memory" setting is actually the size of the page file on the harddrive, not the actual size of the virtual memory (which for Win32 [and almost all 32-bit OS'], is set at 4GB; 2GB for kernal level data, and 2GB for the process itself]. All the setting really adjusted is the size of the Hard Drive dedicated to paging.
As the page file fills (and becomes fragmented), performance suffers (Even the most optmized Best Filt algorithm eventually starts to suffer when limited space is avaliable), so 1.5x system memory is usually a good setting. 2x also makes sense. Really, as long as you have the HD space, its not that big a deal.
I believe Windows defaults the size at 1.5x System RAM, and grows the size of the page file automatically anyway, so unless you are running something REALLY memory hungry, you're probably best off leaving teh setting on automatic.