your phone memory not enough to save all imp SMS,archive it into your gmail account using psiloc.
Psiloc GSync allows you to automatically archive all your SMS messages on the Gmail™ account. The SMS messages appear as e-mails, are marked as read and labelled as SMS. Messages exchanged with same people are grouped into threads. You can configure how often messages are archived.
so no worry for sms memory
click here to archive: http://shop.psiloc.com/en/Application,262287,Psiloc+GSync