Virtual Memory Size

90% of the time you can set VM to 50-80MB just for the crash dump file when windows bluescreens.
If you use Photoshop or something simular to do 3D modeling 8gig should be fine.
Beware, some games specifically state you need to have a pagefile/VM set to at least 2 gigs.
I use Win 7 64-bit with 4gigs physical ram and set VM to a minimum 2gigs and max to 2gigs so it keeps the pagefile non-fragmented and have had no ill effects. But all I do is game and surf the web.
Hope this helps you.

wykydvamp is right, if you use photoshop, at least 8gb, 12gb to 16gb would give some increase on the performance but not that much, abou games, the majority of steam AAA games ask for 4gb to 8 gb to work properly. In my case i work with 8gb, since i do have now an ssd i gonna set the VM off cause for all i have ready so far ssd´s dont have the need for it(but you can set the VM on a side hdd just to keep your games happy)

You can change it in the performance options if its the same thing I'm thinking of, but mine is 2432mb, set automatically by windows and I'm running 16gb of RAM and SSD.

Since Windows Vista, it has been recommended to never go under 1GB (1024MB), to prevent any chances of the system becoming unstable. You could find this advice if you did a simple Google search, even if you looked up about SSDs, they say to always have at least that amount on the SSD.

Any more would help other programs that actually can use extra, but in most cases it isn't worth it against storage space.

Disable it, entirely. Set it to ZERO. You will be glad you did.