I bought 4 energizer 2500 NiMH batteries at best buy for about $14 (no charger includes, I already had one). They last about 4 hours for me, so in total I have about 8 hours to play with my gp2x without recharging anything.
About deciding whether or not to get the gp2x:
I love my gp2x, but I know that many people bought theirs expecting it to "just work" like nintendos handhelds do. This is not the case with the gp2x (or gp32 for that matter). To enjoy the gp2x you will have to get used to waiting for updates for emulators, playing half-complete emulators or games (either that or limited yourself to the few complete ones), and tinkering with things to get them to work. If you really want to get into the spirit of the gp2x you should probably try your hand at making something; if you're a programmer making a game, if you're a good artist then maybe making a skin or icons for someones emulator/game, if you're a writer you might want to try your hand at making a visual novel (although this would require at least a basic level of programming). If you can't do any of those things, well, you can either talk on the boards to people and wait for people to make things or you can go and learn how to program (or how to write or how to draw).
I love this stuff, and I love my gp2x.
gp32_console
R.I.P.