You can use VisualBoy Advance. The emulator can stretch the image to fullscreen. It's rather slow though ... so you will have some frameskip.
That emulator is not slow,it's full speed with excellent sound,don't use framsskip set it back to default settings and set the cpu to 900mhz,by doing that it runs like real gba system.
Believe me i was astonished that it ran full speed,every game i play it runs perfect with no slowdown and one thing this is one of the best emulator on pandora.