I have the 733 and have had no real issues with X. From what I've read, however, your speed problem isn't really linkable (is that a word) to your processor. In fact, X runs great on my old 1998 Powerbook G3 - not as fast as the 733 but it is not that big a deal as to make me not use OS X on it.
I have done everything I could to recreate the problems described by others - impossibly slow finder, screen redraws, etc - and have failed. I guess I'm one of the lucky ones. And that's just it - it seems more a function of luck than an individual's set up. People have told me that they've done their install of OS X in exactly the same manner as I have on exactly the same set up, and while I cannot guarantee that they are telling the truth, some of them have had major problems while I and others have not.