I have an Intuos 3 at home, an Intuos 4 at work and I had to buy a Bamboo Pen the other day while I was away on a business trip because I forgot my pen at home and I just can’t use a mouse.
From my own experience the best of the three is by far the Intuos 3, it has a great sensitivity when it comes to pressure levels, I don’t think anyone can take full advantage of the 2048 levels on the Intuos 4, 1024 is more than enough.
The Bamboo is rather limited in functions, I seldom use the buttons at the tablet, but I have the two buttons at the pen highly customized for each application I use, including Safari and Mail, not to mention the heavy stuff (Illustrator, Photoshop, C4D, Premiere, etc) in which the functionality of the buttons really comes to life, the Bamboo tablet is very limited in that aspect, basically you can program a function for each button and stick to it throughout any software you may use.
The Intuos 4 pen nibs wear out incredibly fast, and they have a more rigid sensation against the tablet.
I’d really try to find an Intuos 3 if I were you (eBay) and if none if none is available then definitely go for the Intuos 4, it seems like a lot of money in the short term, but it will pay off itself if you plan to start doing professional reotuching and illustration, the Bamboo can’t do that.