Rt clk on My Computer > Properties > Advanced > Performance options > Look down to Virtual Memory > Click change (you'll have to reboot when finished).
One rule of thumb is 2.5 x RAM amount. Many moon ago, I was having trouble playing Wolfenstein, and Activision's tech suggested that, as the hard drive was big enough, I should set max and min to 750 megs. I've left it at about that ever since.
Of course, if you have tons of physical ram, you could try conservative swap-file usage and disable virtual memory, but I'm not convinced this is ever a good idea.