Nintendo DS vs DSi (or something else?)

[Green Eyed Stranger]

Okay, so our oldest is having a birthday in a couple of weeks, and succumbing to peer pressure, we're considering a gaming system for him. We've looked at the Wii, and it seems interesting, but several of his classmates have DS'. So looking into that, I discovered there is a DS and a DSi, but I'm not clear on what the differences are. Further, they don't seem completely compatible (that is not all DS games will play on the DSi).

Stepping back, what would you suggest for a soon to be 8 year old boy? A couple of things to keep in mind, I've never owned a gaming system. I went from playing games on a TI-99 4/A (get off my lawn) straight to PC games. Also, we have two younger children (5 and 3) that will probably either inherit or share what ever we get.

Thanks for your help.