There's a great GBA emulator for PPCs called PocketGBA. On my 6700, it was pretty fast (at least playable with generous frameskip). But it doesn't work at all on the 700wx because of the 240x240 screen.

Using NYDITOT (a virtual resolution app), that is no longer an issue. It works GREAT!!! (same goes for the PlayStation emulator FPSECE, btw).

NYDITOT is a bit expensive, but for GBA on the Treo, it's either this or wait for the 800w. Morphgear's GBA module is not playable on the 700wx.