Stores:
First, like AHunter3, I’d also suggest walking into an “Apple Store” (not just any Apple reseller, but an actual Apple Store) just to see and play around with all the different types of Macs they have. You don’t have to buy from them, but they’re well-equipped with working demo units and probably have knowledgeable staff if you have any questions.
Fry’s Electronics – if you have them in your area – are also authorized Mac retailers. They’re one of the few big computer chains that sell Macs.
When you’re ready to buy, Amazon.com sometimes offers good deals on Macs (especially newer models like the MacBook Pros). They have their own mail-in rebates that few other stores can offer; they also offer free shipping and may not charge tax depending on your state. Their customer service is also exceptional.
If you want to buy used instead of new, another choice aside from eBay might be Craigslist.org. They specialize in local in-area sales, meaning you can usually meet with the person and see the computer for yourself before buying. On the plus side, this means no shipping charge and no non-delivery worries; on the other hand, it also means cash/check-only transactions and (usually) no guarantees of any sort – so make sure the unit is working before you buy it.
Different Macs:
As for which Mac to buy… do you really need the “most powerful” Apple? The best Apple computers go for thousands and thousands of dollars, but very few kinds of people need them. It’d be, IMO, a huge waste of money to go for the highest-end systems unless you actually plan to be using all that horsepower. The Mini is much cheaper and is a good choice for general Internet/office usage – especially if you already have a keyboard, monitor, and mouse.
OSX vs Windows XP:
Windows XP is definitely a dramatic improvement over Windows Me, but it’s still not perfect. Coupled with Internet Explorer and Outlook Express, it’s still very vulnerable to spyware, worms and other viruses. The Mac is much safer in this regard (at least up to now) and you probably won’t have as many worries.
If you don’t have any Windows programs you care to run, if you don’t mind getting used to another operating system and if the slightly higher price is OK with you… Macs would be a good choice.
Oh, and FYI… “CPU” isn’t exactly the correct term for “just the computer part”. The CPU refers specifically to the central processing unit, which is only one portion of the computer box thing. I’m not trying to be anal – I’m only saying this because some people might misunderstand when you ask for a CPU and think you only want the processor, which you don’t. This isn’t so bad in the Mac world where most systems are sold as complete systems, but if you ask for a “CPU” in some PC stores, you might end up with just a little CPU chip and not the rest of the computer. You want the whole 'puter, just minus the monitor and such. Other terms for this might be “box” or “tower” (as in “PowerPC G4 tower only”), but neither are really 100% accurate so it’s probably best to just say the full “computer with no monitor, keyboard, mouse or speakers” phrase – it’s cumbersome but clearer.
InvisibleWombat and rjung:
How detailed are those books? Are they written more for a general audience or for techies? I’m considering switching to a Mac myself, but I’d like a book that covers a bit more than the bare basics.