Just to clarify, the Bold has 1GB of on-board memory, which is the maximum size you will find in any BlackBerry devices. By default BlackBerry devices save emails for 30 days. But if you prefer, you can change this by going to Messages/Options/General Options/Actions/Keep Messages.

Otherwise, if you would like to use a phone with bigger memory, but keep the BlackBerry functions. On certain Windows Mobile or Nokia S60 smartphones, RIM offers a software called BlackBerry Connect. Which will allow you to use BlackBerry push email on them. You can contact your service provider for more information on the availability.