Well, I can't say that I know the exact solution to your problem but I can tell you how I have mine setup.

I am running Vista x64 as well, and all clients have to have vertical sync turned off and make sure that the Aero toolbars is turned off as well. I found that even with the one client running maximised, the aero toolbars in the background would cause it to run terribly slow.

Hopefully your problem was as simple to fix as mine.