Well, it looks like the cause for the choppiness in this x64 build is not the Glass effect itself but the window animations.
I went into the performance options and turned off the max/min/close effects. I also changed the power profile from "Balanced" to "High Performance" because I didn't want CnQ kicking in, in case Vista includes an AMD CPU driver that supports it out-of-the-box. Other than that, I turned UAC off as I did in 5365 because that flicker was driving me insane (it's not as bad in this build, but I thought it would have been fixed by now).
With all the little tweaks applied, things are working pretty damn well. Everything's popping up instantly. Looks like XP's going to get neglected for a while again. 🙂
Got Office 2007 Beta 2 installed, and set up Outlook to connect to the Exchange server. Love all the new featured and UI changes in Outlook, and the fact that all of Office 2007 finally supports Glass in Vista.