BE CAREFUL!
While the Ansmann Digital 2600 have highest capacity, they will suffer the same problem most high-capacity batteries have with the gp2x: They won't last long, as they are NOT able to provide the power needed for the gp2x.
The PHOTO ones are able to provide more power, even though their capacity is NOT as high as the Ansmann Digital 2600!
The Digital 2600 are fine for standard devices which don't drain much power and once and will last a loooooooooooooooooooooooooooong time.
But only if the device doesn't need too much power - and the gp2x does.
EDIT: Ansmann PHOTO have Ansmann PHOTO printed on them