I know but there aren't option keep aspect ratio. Every screen on RTG is scalling to full without aspect ratio.
Example:
320x256 native mode screen scalled to 1280x1024:
http://img841.imageshack.us/img841/7861/nativemode.jpg
320x256 UAE RTG scalled to 1280x1024:
http://img688.imageshack.us/img688/3263/rtgmode.jpg