You've only got so much actual physical memory. Anything else is stored as a swap file on the hard-drive.
You can increase virtual memory as much as your hard-drive will allow, but you're not actually increasing the amount of memory you actually have, just the maximum size of the swap file.
And as I said the swap file is exactly that - a file where data is swapped between memory and the hard-drive, which is a lot slower than regular memory.
If you've got things like text documents then virtual memory is fine, because you'll only ever be editing one at a time, and you don't mind a short wait while the data is swapped over when you switch between them.
However if you're using Flash/Shockwave stuff which is constantly going to be reading & writing, then virtual memory isn't going to be good enough, because it'll be too slow.
So, you want to do what mole said and buy some more memory.
