As most OS X users are probably aware "virtual memory" (or paging, in OS X's case) is always "on," as is the case with any modern operating system. It is an integral part of the kernel's functionality. As a result, every time OS X is booted a swapfile is written to the partition where OS X resides. When many apps are open and free physical RAM becomes scarce, more swapfiles (80MB each) are written to the disk at need. As far as OS's go, OS X has a particularly robust paging system--but it does suffer a speed hit due to OS X's default configuration of writing the swapfiles amidst the files on OS X's partitions. Inevitable file fragmentation forces the swapfile to be written to and read from all over the disk, imposing the seek-time limitations of the disk on OS X's attempt to swiftly page data to and from the disk. The solution: create a dedicated swap partition (this is the way Linux's swap system is setup by defualt) for OS X to page to (on a separate physical drive, if possible). In searching the web for information dealing with this alteration, I found a number of sites with prodcedures to follow. The one I felt most comfortable using as a guide was setup by Andy Moraitis. In looking at his procedure and through back-and-forth e-mail with Andy, it's clear he has a deep understanding of the mechanisms of OS X and its paging system (and that of its predecessor, NEXTSTEP). And lucky for those with a mind to increase OS X's performance, Andy has recently put out an even more detailed version of his procedure, which you can get from his iDisk. (Note this guide is in Tex-Edit Plus format--download this editor from VersionTracker.) Truly, if you're even remotely comfortable with using the the OS X Terminal shell, this modification is time well spent.