You know something... I;m about to make a guide called "The smart Mac Buyer" or something like this, because since I was very young I hve been buying profesional and expensive electronic devices (starting with midi and electronic music related) and specially when you live in a society where there is not much money to spend and the one you spend have to be in a very smart way, that creates some kind of sense of what you need, how long is gonna last and how use full is gonna be.
For example, I bought my Powerbook G3 500 (top of the line) just one month after it was discontinued. I wanted to have a powerbook bad but I knew that most of the people were about to sell them to get the TiBook (that for me still inferior in capabilities and functionality/price relation). So, I got mine for $1400 with 2 bateries and 256MB ram.
The same thing with my G4 400, the last computer with audio in (just like the Powerbook).
And that is after getting keyboards, midi interfaces, etc.
You have to look up for what is gonna become a clasic. Clasic means a uniti that years later would be compatible and worth it, like Newtons.
For example, the top of the line iBook looks like a winner to me, just like the top of the line iMac.
I always buy the unit with more fetures of that generation, the one that is gonna be compatible for longer time.
For example I would never buy and old iMac without firewire or the old generation of iBooks. From the last generation iBook I would get the top of the line Graphite, because it has one of every single port, video out put and is a good looking device, that means that would be easy to sell later one. I have a friend with a blue iBook that only came with one USB port and no one wants to buy it.
Be carefull with thouse units that have not every single features. I call them "incomplete mutations" because they are not the best of that line. If I ever get one, I would use it and sell them while they still in production because they would be very hard to sell later and they would not furfill the expectations in compatibility and durability.