Question: I know that the iPhone SDK requires Leopard, i.e. ~900mHz G4+ w/ 512MB RAM. And I've used XCode in the past, so I have a good idea of how that performs. But I've never used the iPhone emulator included with the SDK, and I'm wondering what kind of CPU/memory resources it needs to run smoothly.

I would just download it and see for myself, but my (ancient) iBook can't run Leopard. Before I go buy a new machine, I'd like to get an idea of the **slowest** system that can run the emulator at a speed close to that of the hardware.

Answer: 
RAM is probably the biggest constraint. That's not an iPhone SDK limitation as much as it is an Xcode limitation -- it likes at least 1.5GB to itself when debugging but it works okay at about 1GB as long as you stop the debugger when you're not using it.

I have run the SDK on an old iBook G4. So CPU speed requirements aren't extremely high -- although you may want to edit text in an external editor since Xcode's autocompletes and syntax highlighting are too slow below about 1.6Ghz.

Realistic recommendation: *any* Intel Mac with at least 2GB RAM.
simulator is not the problem, instrument is.

if you just want to run a iPhone emulator, then system requirement doesn't matter as long as you could run leopard.

if you want a production environment, it should at least c2d 2.0 2G ram and still wait a lot for debugging.