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.
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.