For a while now I've felt that the best 'bang for buck' that Apple sell is the iMac range.
The laptops are wondrous machines but they are expensive and they do top-out in terms of performance at a certain point, and connectivity to other devices can become limited. The Mac Pro/towers are fabulous (I used to have a dual 2.5 G5) but you do pay a lot for that expandability and engineering - a quick look on eBay will show you that both the laptops and the towers can still command near-new prices, especially as Apple owners will often tend to get Applecare, keep the original packaging and use Apple-approved type parts and upgrades.
The Mac Mini unfortunately has increased in price as the spec has improved and I now think it's quite an expensive way to get a Mac.
A second hand Intel iMac, or even a low-end brand new one, gives a good mix of reasonable CPU, upgradeable memory (to a point), plenty of scope for FW800, FW400 and USB external connections as well as a potential internal hard drive upgrade if you get really desperate. It will happily drive an external monitor, something that in my experience people new to Mac don't realise (they think that it's all about the built-in screen and nothing else).
It's obviously more of a commodity machine than the towers and that's reflected in the price - you can get an awful lot of iMac for the same kind of price as the entry-level laptop. And if you can find a second-hand or factory-refurbed model, even better!