I'd be really curious to know what reasonable minimum specs would be, actually. But that's a moving target anyway.
The really big "recommended" right now is having two cores, since the AI thread can use the second core and generally runs as fast as it can and would really compete with the main thread for cpu resources on a single core (this is not an issue if you're non-host in a multiplayer game, of course, since only the host runs the AI thread). We don't tend to spend much time optimizing the AI thread because the main thread is what blocks most folks and speeding up the AI thread has a very very minimal impact on main thread performance, etc.
Though Chris did just make some pretty significant improvements to the AI thread performance (there was a function that was taking 60% of the time on that thread, and now it's basically off the radar), so even the single-core scenario should be much better than it used to be.
I've heard tell of folks trying to play AI War on an Asus EEE PC or something like that, I'm curious to know if that's doing any better now
And then all I have to do is find a .NET implementation that runs on a toaster; might have to start with one of the 4-slot ones though