Dave's got a point, but allow me to further elaborate on what he means to tailor his statement to the current situation of computers. I will emphasize the two most important components in a computer to graphic/multimedia users: the CPU and the graphics card.
There have been many recent advances in technology, and we are on the verge of seeing quite a few more in the very near future. For example, you may be hearing the buzzwords "dual core" and "per watt efficiency." What this means is that the industry at large is moving towards multi-core processors (more than one processor unit in a single CPU package) and making it so that a lower clock speed processor can do the equivalent amount of work of a current, higher-clocked processor, therefore wasting a lot less electricity and generating less heat.
Intel is 3-4 months away from completely tossing their Pentium 4 and Pentium D processor line in the garbage in favor of lower-clocked, electrically efficient, multi-core processors (the highest speed on their upcoming processor family is only 2.66 GHz, versus the fastest Pentium 4 or D which is running at 3.73 GHz). AMD has been using this high-efficiency, lower-clock-speed profile for some time, and we are about to see a similar de-emphasis on the "faster is better" from their camp when they move on to using DDR2 memory.
What's going on in the graphics cards is a somewhat different story. There has been a strong push for using multiple graphics cards in a computer system. nVidia developed the SLI (Scalable Link Interface) technology for dual graphics cards, and ATI has countered with their CrossFire technology. Each technology has the capability of splitting the load of renderin on-screen graphics, which leads to an increase of productivity. Both technologies perform very well indeed, but the major drawback is that video cards, as they become more and more powerful, they are also becoming less efficient. Many top-of-the-line graphics cards can consume up to 150 watts of power on their own, which is more than any CPU available today, and a dual-graphics card setup, obviously, will consume twice that amount.
The rest of the computer is fairly easy to figure out. In terms of storage (hard drives, memory, DVD/CD drives), bigger and faster is better.
In terms of Mac's, the move to the new Intel processors is confusing a few things simply because there's a lot of software optimization left to do. Buying anything with a G4 or a G5 processor these days is generally not recommended, just for future-proofing purposes, but buying a new Intel Mac, as it stands, doesn't quite show it's strength and superiority just yet because 95% of the software out there is still designed specifically for the previous generation of Macs. Also, if your situation isn't completely desperate for a new system now, I'd wait to see what happens to the rest of the Macintosh line (as it stands now, only the iMac, Mac Mini, and the MacBook have been switched; we're still waiting on the PowerMac and the iBook).
As far as choosing between a Mac and a Windows machine, I think it's rather simple for a graphic/multimedia user. It's a matter of preference of interface (do you like the simplicity and control of a Mac, or have you been accustomed to the ways of Windows that you see no reason to switch?), and it's also a matter of whether or not the programs you use are Mac or PC specific. 3dsMax, for example, can only be had on a Windows box, whereas Final Cut Pro is only for Mac users. These situations can pretty much make the decision for you.
I have in this tech forum price guides for custom-built PC's. They haven't been updated recently (probably something I should do for Spring), but they will give you an idea of what can be had for a certain dollar amount. Also, look outside the usual suspects for computers (Dell, Gateway, HP, etc.); Maingear PC, for example, is a botique builder where they give such specialized service, the person who makes your system is the same person who gives you tech support and fixes hardware problems (though, there is no on-site service). Lastly, a good source for reviews on custom-built systems for the demanding user is [H]ardOCP
If you have further questions, feel free to contact me.