I've had these Mac's:
PowerMac G4 800MhZ
PowerMac G4 Dual 1GhZ
Mac Mini G4 1.25Ghz
Mac Mini Core Solo 1.5Ghz
My conclusion is that Mac's can be fun. However, there are a few things to point out.
1. Don't even think about running OS X with less than 1GB of RAM (2GB is even better). It just sucks!
2. Don't worry about having the fastest Mac. Apple constantly releases something just a little bit faster than what you just bought, two days after you make your purchase.
3. The GUI seems to be a little sluggish, when compared to Windows. However, you'll quickly become accustomed to it.
4. Whatever you buy, new or used, make sure that it includes the iLife suite. iMovie, iDVD, iPhoto, and etc. are lots of fun and help one to learn the OS and decide if they want to continue with it.
5. Have fun.
6. Oh, I think that dual booting is a pain in the neck, so I wouldn't go into this with the idea that you can replace your Windows machine with a Mac. We have something for people that use more than one OS, it is called a KVM.