I have no idea what my limit is (from a quick question to an IT guy, probably 2 gig). I, as others suggested, use archiving rules to ditch most stuff in 6 months. I routinely e-mail stuff with screen shots and such eating up a meg or so and those stay in the Sent folder for 6 months. However, I do save all conversations about work in separate folders (stripping out any attachments) going back 2 and a half years.
You know why I don't have any memory issues? I don't use my work e-mail for personal use. If I talk to someone by e-mail it is on my yahoo account. Oh and I warn anyone that I correspond with that please keep any conversations as if we were in a public area. Why? Well since this thread has turned decidedly IT, the bastards are notorious snoops.