I'm going to get a DS myself very soon. I'm gonna get my hands on one of those Mario Kart bundles and never put it down, i love that game.. and the added ONLINE gameplay will kick ass. The Mario Kart package also comes with a red colored DS that only comes with that bundle. (comes with stick-on decals too and a few other fun things hehe)
I'm also going to pick up Mario 64 for DS. They changed so much stuff from the original. You now start as yoshi instead of mario and can eventually switch back and forth btween Yoshi, Mario, Luigi, and Wario. There's 150 stars now (instead of 120) and a HUGE amount of minigames. It also has a multiplayer mode! A bunch of the encounters have changed and the new abilities of each playable character will make playing the old levels different fromt he original from the 64 version. There's also quite a few completely new levels thrown in as well. The graphics for the DS version are BETTER than the original for N64!
I'll probably pick up a few RPGs as well. I was thinking of going for Shining Force and FireEmblem: The Sacred Stones.
You have the ability to play Advance games too with the DS. I was thinking of getting The Ledgend of Zelda: A link to the Past (Four swords) from the advance, but then i saw that they were going to come out with it for the DS, and i would rather get that version since it would most likely make use of the 2nd screen, and the advance version wouldn't.
I'm already praying that there will be a Zelda 64 and Starfox game!
There's so many other features that the DS has too. For example there's a camera/video camera that works witht eh wifi so you can talk to people who a are nearby and stuff. there's messaging with text and an area to draw pictures and send them off (can play hangman or something). There's no analog stick on the system *BUT* there is a little thing that you can attach to your thumb that you can use combined with the touch screen that works as an analog stick. There's also a memory thing that you can get to slap movies and music on to.
Every time i think about it I get this retarded smile on my face.
Edit: I almost forgot! For games like Mario Kart for example with multiplayer you can play with friends nearby with a DS who don't own the game! You will download a version to them and they can play with you. I'm pretty sure this only works when in the sam room, or something. I doubt it works on the internet. But its still a kick-ass feature.
The DS also has Voice response capability. In one game (i forget which one it was) you could LITERALLY cheer your character on, and he'll fight harder for you.