hi there
The ds lite, is basically a slimline version of the original nintendo ds console. It can play the old gameboy advance games, plus the nintendo DS games.
the dsi is a newer member of the ds family, see this link for more information:
http://en.wikipedia.org/wiki/Nintendo_DSi
other than a few new features, I personally don't see the point in it, with the dsi, it loses the gba slot. The dsi xl is a larger version of the console, aimed at older gamers.
One that I would possibly go for later on is the Nintendo 3DS, it is supposed to have loads of upgrades, and a fantastic launch line up:
http://www.officialni.../article.php?id=17899
http://www.officialni.../article.php?id=17908