Controlling video resolution in Ubuntu Linux

I am a absolute Linux newbie. I installed Ubuntu Linux 9.04 on a Sony Vaio vgn-txn25n and everything went perfectly. It detected my video and auto set it to 1366×768 resolution perfectly. I temporarily changed the resolution to 1024×768. Prior to the change it gave me a message that I think referred to a “virtual something”. Now I can no longer go back to my preferred 1366×768, so have lost my widescreen. I can’t seem to find anything that will help me with what to do. Any help available from anyone out there? Thank you in advance.
Ron Tate