I never thought about this, since Classic is pretty unobtrusive (you never see it as an application, just as a framing environment for Classic apps). I rebooted this OS X box today, and when I logged in, I stopped the Classic environment from starting (usually I load it at login time). Now as you can see, the load average is quite low on the graph.
So an Xserve might not come with a performance penalty after all.
I thought Classic was supposed to “sleep” when it wasn’t in use.