Well, for the vast majority of things, the 312 will be fast enough. Experientially, they feel very similar (I have both). So far, apps have been designed well for the slower processors and don't beg for 624 MHz. Now there may well be apps, especially games and multimedia, that do take advantage of the faster processor in the future. So if you're worried that your Axim won't be up to the task a year from now, the 624 might give you a bit of insurance. If you're not a heavy gamer and movie-watcher, then the 312 should suffice. I think the 624MHz processor will make much more sense in VGA Pocket PCs, when they come out, since those demand more not only of the graphics processor, but the main CPU itself.
-------------------- Lisa Gade Editor in Chief, MobileTechReview