Is there a location on the web site that describes what the minimum/ideal specification of a development machine is ?

Not that I'm aware of... Essentially any machine that will run Linux could be used, but if you want me to pull some specs out of thin air, I'd say a decent Core 2 Duo and 2GB of RAM would be the lowest I'd go out of choice. It really comes down to how much time you want to wait around compiling/packaging. You could develop on a machine you built out of free scrap parts, but it wouldn't be much fun. The one thing to note is that if you want to use the Maemo VMWare image, rather than setting up the SDK on your computer yourself, then you're going to take a performance hit, as it's running a second OS on top of the first one.
So, essentially, whatever you have will probably work, but if you're buying a new machine with that intent, the more cores and RAM you have, the better.