One thing to note that if you have over a GIG of ram. Make sure you have the latest version of FSaurtostart or get the memory defrag patch. Default Fsautostart had a bug where it wouldn't detect over 1 gig of ram to defrag (not that it would cause an issue - you just won't be defragging all your available memory)
Defragging system memory, if that is in fact to what you are referring, is completely pointless. Defragging a physical hard drive can help improve performance because it puts all the pieces of a file in consecutive sectors on the hard drive. The physical heads and arms of the drive then don't have to jump all over the place reading a single file.
RAM write speeds are about 20 times faster than a hard drive's burst transfer rate (and that's being conservative, comparing PC3200 DDR400 memory to a SATA150 hard drive) and 40 times faster than a sustained hard drive transfer rate. RAM allocation by its very nature results in fragmented RAM contents, but that is of zero consequence because there is no time penalty retrieving data that is not stored sequentially.