Lyte Edge said:...The company selling a car power adaptor?
The thing to remember about getting a PSP, haunts, is that you're going to have to spend another $100 or so to get a good size memory stick duo; if you want to watch movies, you're not going to want anything less than 1gb. As for games, obviously GTA is the one to get, but don't forget about stuff like Darkstalkers Chronicle and Guilty Gear XX #Reload. Plus if you can get the right firmware, you can use all the emulators.
The DS is going to be cheaper and there are awesome games like Meteos and Castlevania available, so both systems are worth looking into.
Although if you just want something mess around with in the car for a little while, screw them both and go pick up a GBA SP or Micro and some cheap games instead. I wouldn't bother buying a DS or PSP unless you really think you're going to be playing them a lot after your trip is over.
Why spend extra $$ for another memory stick duo when the gigapack is already out?
