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.
The simulator runs pretty well even on the slowest Mac in the lineup, the base-model MacBook Air with the hard drive (non-SSD). It runs applications *far* more smoothly than they run on the real iPhone.

The rest of the development process, though, benefits from a bit more horsepower. An SSD (optional in the laptops) or a 3.5" hard drive (iMac or Mac Pro) is a huge benefit, and you'll want a decent amount of CPU power for quick Xcode responsiveness with autocomplete, etc.

Most of all, you'll want as much screen space as your budget and portability needs will allow. It's pretty cramped on the 1280x800 screen found on the 13" MacBook and MacBook Air. Ideally, your minimum is a 15" MacBook Pro or a 20" iMac.

I recommend a CPU at or above 2.0 GHz and at least 2 GB of RAM. Ideally, it's at least 2.4 GHz and 4 GB of RAM.