I don't want to start suggesting which model to buy because I'm still quite new to the world of mac.
What I will say is, whatever you decide to spend your money on you won't be disappointed. I've had an iBook for about a month now and have virtually stopped using my Windows box.
I'll agree that they aren't cheap but when you start using your mac you'll realise it was worth it. The ilife package that comes with bundled is also great, the only other piece of software I've bought is mac:office so I'm compatable with windows users, apart from that everything has been freeware or supplied with it. Meaning I can save my money for some of the more expensive, creative stuff.
I've not done much development but have just found a little app called xcode which looks great. I thought I had to buy it seperately but found it hidden away on the os x install disc last night. I'm going to have a crack at "Hello World" in this as soon as poss.
One last thing, if you're worried about learning how to use a whole new os, don't be. Any time I've wanted to know anything I've just typed it into google and got my answer in minutes, top stuff.
EDIT:: Then ofcourse there's Unity. I've yet to have a go at making anything in it but from what I've seen and the docs I've read I'll be a very happy chappy when I do.
I AM A MORON
and won't change my sig until I get off my oversized behind and finish a project.