It will most likely be a mkII. If it has the flat joystick button and the sticker on the back has korean writing on it then it definately is a mkII.
The mkII has a different screen which shouldnt have scanlines like the mkI but also loses some of the crispness of the mkI screen, also the mkII screen has weird brightness/contrast issues if not viewed from a perfect angle, whereas the mkI screen looks the same from nearly any angle.
MkIIs also dont overclock as high as mkIs on average but you may get lucky, my mkI overclocks to 279mHz fine but 280mHz makes it crash.
The joystick mechanism in the mkII was rotated 45 degrees before being soldered on giving it greater accuracy and less bias to the diagonals. And a revised version of the cap was fitted and I believe the joystick shaft was also shortened.
So there you go, the differences between the two... either way prepare to have fun.
edit: forgot about this one.. The mkII does not need an extra power supply for the breakout box as it supplies the breakout box with power from the gp2x unit itself. A mkI with a breakout box neds two power supplies, one for the breakout box and one for the unit itself.
