Don't quite understand the question fully.
If you're talking about the 'swap file' or virtual memory (vm), it's easiest to let the system decide what's best. Btw, this is irrespective of the the hard drive's built-in cache/buffer that's independent of virtual memory I think.
Or, manually set a vm figure that's equal to 1.5 times your system RAM. I wouldn't set it to more than 2Gb though, no matter how much system RAM you have. G