What is the minimum hardware needed to run Xcode 4? I understand from the Apple developer site that Xcode 4 requires an Intel Mac running Snow Leopard, but I can't see any advice on hardware spec (other than Intel).

I imagine it could run on an old Mac Mini with 1GB of Ram, but that perhaps I'd be pulling my hair out before too long.

What is the minimum sensible spec to run Xcode? This would be for developing iOS apps, so I'd be running the emulator too. It is only for home use and my own education, so I don't have a big budget, so I'm looking to buy off eBay or similar. As I already have spare keyboards, mice and monitors I'd probably get a Mac Mini.
1GB of RAM will kill you: frankly, I can't imagine that it would meaningfully run. I'm currently using a 2009 MacBook (not Pro) with 4GB of RAM, and for the first two or three hours Xcode runs pretty well. 

After that, though, it gets slower and slower. On a pretty small project Xcode will be using multiple gigabytes of RAM with the OS swapping 2-3GB to disk. Before long I not only have to quit Xcode, I also have to restart the machine to get things back up to speed. (To head off potential commenters, no, it's not something else on my system, it's Xcode 4: no other piece of software, including Photoshop operating on a 20+ GB file, fails to clear up the swapping speed problem after quitting the app.)

I can safely say, both from my experience and from talking to others, that 2GB of RAM is the bare, bare minimum, and it's not going to be a good experience. I'll be replacing this system soon, but in the meantime I'm upgrading to 8GB because I'm losing too much time to only having 4.