Everyone has lots of good suggestions! I'm glad that everyone is as excited as ever to be a Mac user.
I have a few things to add, though:
1. If you can afford it, I would buy a new Mac. This has many advantages: first, you know that you are going to get a high-quality built machine with parts that are factory-fresh and haven't been used. Along with this comes the standard 1-year warantee (which I think is too short -- Apple should give at least a 2 year warantee). Basically, you can be assured that you won't one day in the near future boot up your computer and find that one of your components is fried. The second advantage is that you get all of the latest software preinstalled: OS X 10.1.1 with all the security and installer updates, iTunes 2.0.3, AppleWorks 6.2.2, Mac OS 9.2.2, and if you get an iBook or iMac, you will get Cro-Mag Rally, Bugdom, Nanosaur (and maybe Otto Matic) preinstalled as well as AppleWorks. ALL new Macs come with iMovie and iTunes pre-installed, and if you get the top-of-the-line G4, you'll get iDVD as well. Lots of applications = lots of usefulness. Third advantage: they will last longer and be better able to deal with the latest applications and system software. You may be able to buy a used G4 for a lower price, but then again, it's probably not going to last as long as a new top-of-the-line Mac. The only exception to this rule might be dual-processor G4s, since OS X does take advantage of both processors.
Also, you also expressed interest about OS X. Used Macs may NOT come with OS X. If you get a new Mac, you will be guaranteed to have OS X pre-installed for you.
2. You said that you can't stand the size of the iMac's screen, yet people still recommend the iBook. I should say that, yes, you probably get most bang for your buck from the iBook, but you also get a 12.1" diagonal screen (compared to the iMacs viewable 13.8" screen). If you can't stand the iMacs, I'm certain you REALLY won't like the iBook's screen. However, if you can live with that small screen, you can get the combo drive iBook (best value) for $1699 that has a 600 MHz processor, 128 MB RAM soldered onto the motherboard, and a 100 MHz system bus, not to mention the DVD playing and CD burning/reading. It definitely is a good system, but it is VERY limited in expandibility. You can basically only add RAM and an airport card. Everything else has to be external. The G4 towers would be much better if you want to add internal parts and stuff. The PowerBook is a VERY good deal too, and it has a VERY large screen for a laptop (15.2" viewable diagonally), and it packs quite a punch -- but again, expandibility is limited. It seems like the best for you would be a G4 tower and a third-party monitor.
3. If you do decide to go for a new Mac OR a used Mac, wait until after the MacWorld Expo this January 7-11 in San Francisco. This is VERY VERY IMPORTANT. Why? New PowerMac G4s and new iMacs (supposedly the form factor is going to change, so you might like the new ones
) are almost definitely going to come out. So if you're splurging and getting a new Mac, you can get a better computer for the same amount of money, OR you can get one of the old new Macs (the current G4 towers) for less money, and basically get the same quality. Often, Apple itself has some old towers stocked up in inventory, which they offer deals on. Many Apple retailers also offer good deals on recently-made-obsolete Mac. It's a win-win situation to wait for MacWorld. (NOTE: Also of interest -- PowerBooks and PowerMacs might come with AppleWorks pre-installed as of this MWSF, which is a BIG BIG plus. And there might even be a new 19" flat-panel monitor, which is really nice if you want that space, and also nice if you want to get a 17" for only $599. )
Hope this helps. ANY Mac you get will be a good deal, and will last a long time. Just make sure you look at what you are going to do on your computer, and see if any of them are not suited to do that, or if they are overkill. Once you rule out those Macs, choosing is much easier. Basically, if you're only browsing the web and doing word processing, I would sincerely NOT recommend getting a G4 tower or PowerBook. That would basically be a waste of computer power and money.
As to your last point, the Mac offers many advantages. UNIX apps can now run on OS X, meaning as soon as OS X came out, there were instantly MANY more Mac developers. Maya, one of the apps that was mainly used on UNIX (and Windows) was ported to Mac OS X, signifying that Mac OS X is really a viable platform. You get a much more intuitive platform, and you don't get forced into the crap that Microsoft slams down on you, like all the advertising in Windows XP. Plus, even though having a command line in Mac OS X poses a little more of a security threat, UNIX still is VERY secure, and is much more secure than Windows. You don't get NEARLY as many viruses, and you can even set up the built-in firewall that comes with OS X (provided you download a GUI utility to set it up, or you know the UNIX commands). The best advantage, I found, though, is that you can tinker with the command line and all the components of MacOS X, which makes it VERY customizable. We already have a theme application (MetaMorph X), as well as many tinkering apps (like TinkerTool) that can change the transparency of the Dock and Terminal windows, position of the Dock, pinning of the Dock, as well as many other things that Apple doesn't provide with a checkbox. Plus Mac OS X is rock-solid stable, making your computer uptime fly through the roof.
Even though Macs seem very pricey, you really do get what you pay for -- great quality, long lifetime, not to mention Macs are a joy to use. Plus they are much easier to use, which saves a BUNCH of time. The overall cost of ownership (including repairs, time spent troubleshooting, time before it's necessary to buy a new Mac) is lower on a Mac, rest assured.
Oh, by the way: would you mind coming back here and posting what your final decision is? I'm sure we'd all be interested.