K.S.
We bought the Leap Frog Text & Learn when my daughter was 3. It has some learning games and some other cute stuff like watching the little dog (Scout) dance to different styles of music. Plus, it is only $20 and there are no extra games to buy.
She is now almost 6 and still pulls out the Text & Learn even though she now has a Leapster.
There is a case to be made for buying a Leapster now if you know you'll buy one in a few years anyway, but who knows how the Leapster will have changed by then? It could be like having a cell with a qwerty keypad while everyone else has an iPhone. :)