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: 
The iPhone SDK requires an Intel machine running Leopard, although I believe there is a way to get it working on PPC Macs (but you still need Leopard).

The SDK does not have an emulator, it's a simulator. The code it runs is native so there is no emulation overhead. You will find that even a low-end machine will run your code significantly faster than on an actual iPhone.

(This is why it's super important to test your application on an actual iPhone, particularly for anything graphic or processor intensive).

1GB of RAM and any Intel based Mac will be more than sufficient for both development and any docs/webpages you are likely to be referring to.
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.