the gp2x power packs are just regular GP2X's that have been tested and found to overclock over 266Mhz
If you buy from somewhere else then it's the regular gamble what level it could over clock to, will be somewhere between 240 and 300 though I'd guess the average is around 275.
In the box you should get a USB cable (that few people use - most favour a Card Reader)... eh, what else... hmmm, cant remember, that may be it.
Anyway, you'll want to buy yourself a set of rechargable batteries (2500mAh or more) and charger, an SD card and a cheap card reader will usually suffice (unless you get a 4gig SD card, then you may want a good card reader).