Welcome to the exciting world of macintosh.
Remember that Macs and PC's do have their similarities and their differences, and when it comes to deciding which Mac to get, it's pretty much like deciding on a PC.
1) If you will be just surfing the web and simple office work, you need nothing special: Something with a G4 processor. ( Mac mini, PowerBook, iBook )
2) If you will be doing slight media orginization, ( small amounts of video editing, iTunes and iPod sync, garageband ) you can still use a G4, but if you want optimal performance you should go for the G5, (PowerMac and iMac)
3) And if you will be doing advanced media work, studio sound and video editing, you should only go with the G5, and preferibly with the PowerMac, because of the amount of upgrades it can handle VS any other Mac.
(You can upgrade RAM, HD's and just about anything you can in a PC in a Mac.)
Most of Mac users fall into the second category. And i'm pretty sure you do to. So for you, I would recommend the iMac G5 and the Mac mini. look into the speeds and RAM you will be needing.
And don't forget about some of the sick apps Apple can offer, but most of the things made by Apple can be overpriced.
Oh and one more thing, incase you havn't heard : If you ever have a program for Windows you need to run on a Mac, Apple and Microsoft make Virtual Pc for Mac, which is an emulator than can Windows Xp and 2k.
Jess