Let me admit my biases straight out. I've used Macs for 20 years, and I prefer them strongly for my own use. I've also worked as both a PC and a Mac systems guy, and have built a number of PCs. I can't stand advertising, spyware or shoot 'em up games (I am more sensitive than most to dancing ads and the like). I have always preferred the Mac OS to whatever version of Windows was current at the time, but I certainly like some flavors of Windows much more than others (Windows 2000 was my all-time favorite, and I don't like XP).
One thing to consider is that upper-end Macs are better built than MOST PCs. The Power Mac G5 is a comparable design to an upper end Dell Precision or the HP dual CPU workstations. These machines (whether Mac or PC) are a whole different level of quality from the average desktop PC-just look at the cooling, the way they're put together, the attention to detail throughout. The Mac is actually slightly LESS expensive than its competition in similar configurations.
A legitimate beef against Apple is that they don't offer a whole lot of choice below that level (if you have your own favorite monitor, Apple's offerings jump from the Mac Mini to the Power Mac workstations). Apple DOES tend to aim its machines right at what a photographer would want, while any powerful Wintel box (except for the aforementioned workstations, and also servers) will be aimed straight at gamers, with quite different performance optimizations!
If you can afford a Power Mac, it's probably the cheapest route to a really top Photoshop machine. PowerBooks are among the nicest notebooks out there (but, again, they're comparable to top-end PCs in pricing and Apple offers less choice at lower levels-although the iBooks are very credible).
Windows offers a lot more choice, especially if you're willing to build your own (essentially the only way not to pay for gaming-centered features). Remember that you'll be dealing with buggy, spyware and virus laden Windows XP (it's a shame Microsoft no longer offers Windows 2000, which was much more stable than XP). It may be worth it to many people to bite the bullet and buy a Power Mac even if you don't care about the build quality, just to avoid buying $300 worth of partially effective virus, spyware, ad and spam blockers.
Of course, if you don't have a monitor you're attached to, Apple offers many more options (if you're in the market for a nice single processor system, Apple will gladly sell you one with a 20 inch LCD for under $2000-they just won't let you decline the LCD and save a lot of money, because it's part of the computer).
The one thing that would change this equation is if you also like games. Games just don't work as well on Macs, and, if this matters, you have two choices-Windows or two computers. The same, sadly, holds for using Windows 2000 to get around some of the problems with XP-2000 is nice, but most games won't run. Since I never touch games, I don't care, but anybody who does care needs to consider this and probably just use XP (being careful about the spyware, etc...)