I just put a WD 320Gig drive in my MacBook a couple of weeks back but I took the opportunity to reinstall OSX and leave out most of the extra stuff like languages and printer drivers and iWork trial, then just manually copied the few third party apps that I needed over, along with bookmarks and address books. Everything else is on a NAS.

I wanted the extra space to store FLACs without having to cart an external HD around when I take the laptop from the office to the lounge to connect to my Hifi.