Dave, if you go with a nice Mac setup, you won't be disappointed. It will cost you a little more, you'll have a little less selection in terms of software, and you will probably be less susceptible to some viruses.
Now, as someone who has actually used both professionally for graphic design and advertising for about 20 years,
I will tell you flat out that in terms of using anything like Adobe illustrator, Photoshop, InDesign or any other brand/type of graphics, image editing or page layout program - a PC will perform just as well as a Mac. The software runs the same on either one - and frankly, there's nothing you can do on a Mac that can't be done on a PC. Both platforms handle files a little differently, but the difference will seem insignificant.
Macs have a leg up in the graphics field because they were the first systems designed to be used for those applications - like page layout, photo editing and illustration. The first (and best) apps came out for Macs, and every ad agency and design studio dumped their paste-up boards and graphic cameras for shooting halftone photos and bought new Macs. The copywriters and accountants got PCs, since they were using MS Word and Excel, and the first versions of graphics software for PCs weren't very good.
By the late 90's however, PCs and Windows software had improved significantly, Apple lost some of its focus after Steve Jobs left (PowerPC) and lost market share; as a result, companies like Adobe started focusing on building graphics software for PCs first, because they weren't sure about the long-term viability of Apple. As a result, the playing field got levelled.
Of course, since that time, Steve Jobs returned to Apple, they got back on track, and have returned to a high level of excellence like they had originally. But over those years, PCs have caught up in just about every way, and like I said, are every bit as good for the applications you are using. For some applications, a PC may be the only choice. Many people in the graphics field have only used Macs from the very beginning, and they swear by them - understandably. But being in advertising for over 25 years, I know better than to base my purchasing decisions on TV commercials. The Mac commercials are brilliant, but the advantages are vastly overstated, and they are based on the PC world of 1992, not 2009.
As far as the virus issue is concerned, installing and using good virus protection is the most critical factor. Macs aren't immune - in fact, if you keep up on tech news on sites like slashdot.org, you will find that viruses have entered the Mac world, too - and Apple encourages users to install and use anti-virus software:
http://www.pcpro.co.uk/news/240255/apple-recommends-security-software-for-mac-owners.html
Most Windows problems arise when users load their PCs with crapware, too many programs, too many registry entries - it's a recipe for problems. Regular maintenance will keep you from most serious issues - I have not seen a "blue screen of death" on a PC in over 10 years. Conversely, I worked at an ad agency a few years ago that had a horrible network setup (all Macs) and lost more work from "frozen" machines in two years then I had in the previous 10, using both platforms. I use XP now, would not upgrade to Vista, but may look at Win 7 later.
That said, I am typing this on a Linux PC, which is working pretty well, too. All the software is free, there's usually a free alternative available for most every WIN/Mac program you would want, and it pretty much works the same. File systems are a little different, as is the process of loading/installing software, but it has a lot of potential.
Whatever you do, the major hurdle is the learning curve on new software, should you decide to try something different. It's frustrating when you're thinking -
"by the time I learn how to use this new stuff, I could have done the job using the OLD stuff ten times over." But sometimes you just have to bite the bullet and devote a weekend to learning new software. Good Luck.
Whatever you decide to do, we all know you'll put it to good use. :sifone: