Virtual memory is almost a leftover from the days when memory cost $50+ a megabyte. Your OS will reserve a certain amount of hard drive space and treat it like memory. This was important when the average computer had like 4 megabytes, or even maybe in later years, a gig of memory.
Today though, with most systems now using 4gig or more, you don't really need this function even turned on. You should reserve a small amount though, as the OS tries to save a log to virtual memory in the event of a crash, and actually I don't know or have ever even heard of anyone saying this was really useful for anything.
Anyway, if you have 4 gig of memory using a 32 bit OS, or 8+ gig using a 64 bit OS, you can simply disable, or set this to a very minimal amount, like 50 megabytes, really, most people simply don't need more than that, for any reason.