Yes the GP32 does come in different speeds, Termed overclocked. Overclocking is not possible through direct user control, But done through sofware, So opening your GP32 to overclock is not required. Software such as certain emulators, GPCinema (Movie/video player) and some homebrew games provide a menu option that allows the user to select a speed (limits permitting*) that they wish to run said software at, To allow for smoother gameplay or longer battery life.
*the limt being how far your GP32 is able to overclock without crashing, Which varies to a great degree for unit to unit. That is why Gbax.com (you did get that right) offer two versions of what seem to be the same unit. One is the standard unit that probably has not been tested for overclockability or failed to overclock to 166mhz. The other which has been tested or modified and has overclocked stability to 166mhz or above and so is guaranteed 166mhz.
which you choose really depends on what you wish to use your GP32 for.
Emulation (megadrive, snes, Atari st etc), High quality video, Quake, Doom, Linux etc, Then definately go for a guaranteed 166mhz GP32.
Emulation (master system, Nes, Spectrum etc), Homebrew, Low quality video, Mp3`s etc, Then go for a standard GP32.
But, You could still be lucky when buying a standard GP32 (Pot luck) and end up with one that could overclock way higher, Though the chances are very slim.
My fingers hurt
Trooper