6GB of e-mails seems like a stretch... My mother's Outlook pst files are like 1GB and she receives a crapload of e-mail and a crapload of ridiculous 10MB Powerpoint attachments.
You'd have to be a serious packrat to balloon an e-mail data file to 6GB... Though I guess it's possible. I do agree w/the general idea tho, casual users should definitely be using web-based e-mail, it's just less headaches for everyone in the long run.