Most people that I know that are not happy with the performance of a single machine are running multiple monitors. It is a known issue that multi-monitor support doesn't work very well. It's said to work better in vista, but even that could be less than stellar.

If you run them all on one monitor, you can get excellent results on a single machine. I run 5 clients on a core2 duo, 4G RAM, 8800gtx (hardly over the top), and by limiting the frame rate and detail options on the "clones" get a pretty constant 60fps on the main window every where but big cities. This is with my main window at 2560x1000 resolution, probably higher than most people will want to go. When I've tried more than 1 monitor, my performance went into the toilet.

Also consider that a single machine has some advantages, most notably the PiP type features that keyclone support. You lose that with multiple machines.

I'm actually not trying to dissuade you from multiple machines if that's what you want, there are definitely advantages - I just don't think "performance" is one of them if your one machine is reasonably powerful.