First figure out what you mean by "web developer." This expression is applied to a pretty wide spectrum of services.
The graphics guru at my shop is a gifted designer and uses DreamWeaver. He's very good with HTML and CSS and a master of graphics and can embed his efforts in PHP, if the code's not too bizarre. However, I know better than to rely on him for JavaScript or PHP logic coding.
The chap who runs the server-side programming knows his PHP and MySQL stuff inside and out, and can code his way out of a wet paper sack in HTML and CSS.
The gal that mainly does photographic and graphics work is a Mac-chick.
My background is in Java, and I can do reasonably well with PHP and MySQL. My talents are currently focused on HTML, CSS, and JavaScript which I can apply to make a browser do anything.
I draw those distinctions to emphasize that we could all be called "web developers" but own widely varied skill-sets. What flavor of web development do want to undertake? That said and asked...
The only developers I've known who used Macs for development were devoted primarily to graphics design and photography. There are apparently jazzier tools for manipulating graphics on the Mac. Both my graphics and server gurus use Windows-based PCs. Only our photo artist is a Mac user. The laptop and workstation I use for development are both Linux.
The virtues of Linux and Windows PCs are mainly the large number of software development tools available for either. My guys use DreamWeaver - I use Eclipse (with hope of eventually seducing them away from the Dark Side...both Windows and DreamWeaver). We all use svn (Subversion) for our code repository. I use VMWare to have many different (non-Mac) OS configurations with a wide variety of browsers for testing and resolving cross-browser compatibility issues.
The main agreement among us box-wise is that we all own machines of relatively high CPU rates (my laptop is a 1.6 GHz processor - the workstation is 2.4 GHz). We all use lots of memory - the more you work with large images, the more memory you'll need. If you use VMWare, lots of memory is a fundamental requirement.
You can probably do anything as any kind of "web developer" on any platform. Some tools will be better than others, depending on platform. You'll likely want a fast CPU and a lot of RAM, but you can hardly avoid that, if you buy a new box (just bought a new laptop for my wife, and its basic package was a 2 GHz CPU and 4 GB RAM! - I only use 2 GB RAM on my workstation). I frankly don't see the virtue of Mac for the price, but I'm not the graphics type of web developer; I slice and manipulate images occasionally, but it's not my main thing by a long shot. Heck, I haven't even used Windows on any of my boxes for years...free OS, free programs, free development tools, etc. and very much more secure (my wife's two Windows computers are my major security holes on my six box home network).
In summary, unless you want to do the major photographic and graphics intensive "web developer" thing, I'd stay away from the expense and software headaches of the Mac. Unless you're an uber-geek, I'd avoid Linux (it's still not ready for prime time and has a steep learning curve for the newbie - said the guy who owns 4 Linux boxes and has been playing with Linux since 1995). Stick with a PC running Windows and get it with a fast main processor (1.6+ GHz) and heaps of RAM (2+ GB).