Not entirely sure - although a slower card is a cheaper card.
I use Novatech own branded SD cards - good for f/w updates and playing (www.novatech.co.uk), but can pause for a quarter of a second every so often when playing games - I use a Sandisk 512Mb SD card for games - no pauses. The Gp2x seems to have been tested by GPH with the Sandisk SD cards in mind...