Nintendo DS / DSi

I have an original DS and play it loads, but I'll probably get a DSi sometime for the downloadable stuff.

If you do get one, and want to be able to play GBA games, then get one of the original DS's.
 
Yes!

It has some excellent games such as GTA: Chinatown Wars which makes great use of the stylus. New Super Mario Bros. is also great classic platform fun.
 
I'd buy one just for scribblenauts which comes out in autumn.