Booting from the old hard disk from power on to login screen was 72 seconds. The new one takes 42 seconds. I ascribe that mostly to the improved disk speed (7200 rpm vs. 5400 rpm).
WAY snappier. Astonishingly so. I suspect that much of the caching that is happening is the file allocation and bit map parts of the file system. Even a small cache would do that, of course, but lots of other stuff can sit in cache alongside those items when you have 4 GB of cache.