Thanks Gary, I recompiled xmame.x11 and now I get fullscreen with XV extensions (I had to make some modifications to the makefile.unix). Unfortunately, SDL fullscreen doe not work...YET. Gameboy Advance uses SDL and whenever I request fullscreen I get diagonal squares across the screen like I mentioned before. SNES9x works wonderfully with XV extensions. Now I need to find out why SDL cannot give me a 'proper' fullscreen.
This is a major improvement. My niece thanks you and wishes you a Happy New Year. Thanks again..
P.S. I use gxmame as the frontend. It works nice..but somehow when I click on properties of a game it will not save it (like using a joystick for example). Time to check FAQ and e-mail the appropiate forum.