yeah I am only up to a half a gig on my entourage db, in the current version of Office, and so far it is responsive. But I do not get a lot of email compared to what I expect you get. The nice thing about Mail is that is uses standard mboxes, and this coupled with spotlight makes me think that moving to Mail might be a good idea. I haven't really made use of the project center in entourage that much, but I do use the calendar a lot, and iCal is really not up to it for me. Still praying for the entourage plugin.
another nice app for spring cleaning is OmnidiscSweeper, which can show you readily what is hogging the most space and might be worth deleting. Often times it makes more sense to archive one or two big items rather than spend time trashing lots and lots of small files.