The old rule is 1.5 times, but if you want a constant set size (which is better) then set the Min and Max the same.
The file size does not change, and Windows also uses a virtual memory, so it is always needed, because software is written to use it usually. Yes, anything pulled out of virtual memory is always going to be slower than actual memory using, but whether it would slow down a soft synth loading is something best left to finding a review that tests how those work. It may have nothing to do with virtual memory as long as there is enough memory in the computer in the first place. But Sonar does use the harddrive and that is what the buffer setting is there for. If that is set too low, then perhaps you ought to raise that instead and try that out. And I just set mine way up there (use to say in help to set it at 64, and increase it in steps), well, mine is way above that and over 1024 for the buffers in Sonar.
Otherwise dropouts can occur because there is not enough buffers to work good for Sonar.
And not enabling harddisk reads and writes in Windows or Sonar because if you lose power you can wreck the data on your harddrive, unless you have a Uninterruptible Power Supply for the computer.