Yes, the DS is worth it if you want some fun. But I can't imagine why you would want any more systems than what you already have, how often do you get bored, man?
I'd go for Mario Kart out of extreme Mario Kart loving bias.
For what reason are you thinking of getting a DS? Birthday? No particular reason? I wish my mum was lenient enough to let me own all of that. I still play on a 50Hz TV.