You may like to know that there's a DC DC convertor inside the gp2x for taking battery voltage of anything between 1.9V and 3.6V* and converting it to a usable internal voltage (3.7V), whereas the DC adaptor input seems to have no such convertor and is used "as is". Obviously the convertor will use a certain amount of current to power itself, but you may find that if you use a decent pair of batteries, the overclock is better than an adaptor (assuming most adaptors are 3V).
* - I don't know the exact range, but providing a 1.9V input directly to the battery compartment provided a perfectly working console. Mapping out the current vs voltage, it seemed that 3.7V was the most efficient.