Depending on how much memory you have, this may be the key to the biggest boost in speed under OS X.
OS X is a memory hog, and needs quite a bit to work with -- if you don't have it OS X will gladly use your HD to provide the space it needs. And there is a big difference between a HDs 50MB/s and the RAMs 1GB/s (yes RAM is faster than that, but the FSB isn't).