I agree with Jmax: you can delete a lot of your stuff, which I am pretty sure you will never need.
But if you really need all the data or are feeling lazy, there is a very fine tool to your rescue.
Go to Savemyinbox and install the tool on your machine, give it credentials to your dropbox account (create one if you don't have one) and your Gmail account.
It will back up all your mails to dropbox which you can then access from anywhere.
