I tried building pypy on my macbook pro w/4 4gb of ram and 2.4 GHz i5. After over an hour I had to give up and cancel it. It took another fifteen minutes for the system to recover enough to where I could start using it again.

I don't know what it was doing but there was a python process that was using every bit of CPU available and a ton of swap.

This same machine can build ruby 1.9.2 in under 10 minutes. It took less time to download and refresh all of my installed macports (including postgresql 9.0) than it took to attempt to build pypy.