I'd probably go DS first, then wait a year to save for a PS3. I have a DS and even though I barely play it now, I did when I got it and I plan to when university break comes up -- it has a great range of games. I'd definitely recommend The Legend of Zelda: Phantom Hourglass, Final Fantasy III and IV, Mario Kart DS, and a wide range of GBA games if you don't get the newest model.

The PS3 looks to be receiving a lot of awesome games, some exclusive and some not. But I'd rather wait until a solid library has been absolutely established. That's just me though.

The PSP is also a neat little contraption. Some good games like God of War and Crisis Core. Can also emulate stuff, including PSX games, which is a huge bonus for replaying old classics on the go. To me I don't feel it's PSP game library is that great though.