I'm not a mac expert, but while we wait to see what the pros are going to say, I will suggest it all depends what you will be doing with it. What sort of things do you do? video editing, image editing, just word processing, etc..?
My iMac G5 (see signature) is great and can handle Photoshop CS2 and even Final Cut Pro and After Effects (hell, it even handles Doom III). Of course, multi-tasking these major apps can become slow(er), but still usable. I often work with Photoshop and Illustrator open, plus EyeTV capturing video in the background and it runs fine.
Obviously, a PowerMac is going to handle anything you throw at it, but if you're on a budget, the iMacs are the best bang for your Mac buck IMO (just make sure you get the 20" for the love of cheese, and at least a gig of RAM).
but again it depends on what u plan on doing with it. everything I just said is moot if you're just going to use it for is checking your email.
(also, are you looking at a desktop or a laptop?)