Thanks Dane.
Something else to consider: "archive" older messages that you need to keep but don't need at your fingertips. This keeps a copy on the machine where you're doing the archiving, but removes it from the server and it no longer would count toward any quotas.
I use IMAP but file away messages that I may need to keep for future reference. If I'm away from my house, I may have to wait until I get home to get to them, but I've never had a situation where I needed one of those messages immediately.