It is the Radeon.
I moved my displays to the NVIDEA GeForce 8800 GT - no better.
I then removed the ATI Radeon HD 5770 - problem fixed. Well, not fixed, but identified!.
Interestingly enough, I just moved both monitors to the same card (the ATI Radeon 5770) and restarted....now Aperture works fine. So, for me anyway, it seems that having the 2 monitors on separate cards is the issue (I haven't tried running both on the GT 120). I guess I'll run like this until they have a patch (if they ever discover/fix this). I've left feedback with Apple.