i was once having the same dillemma, and i have a solutin for u my friend.
first pull the vga connecter out of the back of the dreamcast and hold it abt a quarter of the way in so that it isnt plugged in, turn on the dc w/the non vga game in it and let it spin for about 12-15 secs then push the connecter all the way in and the game will ####t on the monitor, ta da
it way take a couple tries the first few times you do it but you will get pretty good at it if you play everything on the monitor like me
good luck-and have fun shooting those hi-rez zombie ba####ds