Trauma Center is that surgery game.
Hotel Dusk might interest you.
Harvest Moon DS as well.
Those three I haven't tried, but I want them.
I do have Bomberman, which has nice multiplayer.
I thought Diddy Kong Racing was ****. Play a friend's copy before you buy it if you're interested. The custom tracks are pretty lame too. (Grey track, and sky background - woo)
I'm just playing Animal Crossing a lot to be honest, Mario Kart is brilliant over wi-fi and when the Pokemon Games come out in the UK and Europe I'll be buying that also.
Guess you'll be waiting for Gears of War DS though.
