For web design, you really don't need anything fancy... It really depends on the tools you use and personal preference. It helps if you have powerful/faster computer if you do extensive work with Photoshop or other processor intensive applications, but it really isn't required.

The whole mac vs pc thing doesn't matter a whole lot for web designers... again, personal preference. If you do graphic design or intensive creative work, Macs are sorta the standard -- you'll see them in most design classes if you attend college. I'm personally a mac fan -- I appreciate that they generally don't crash, don't get viruses, and generally make me more productive -- but that's just personal opinion.

I think you can go either way on this one... A lot of companies work on PCs, a lot work on Macs. As long as you are at least familiar with both platforms, I think that's the main thing.

It sorta depends on the company you work for... and I suppose the programs you prefer. The company I primarily work for is mostly Windows based, since they do a lot of work with ASP.NET (Windows only). That said, I can do basically everything I need to on my mac, and those windows-specific things I need to do (testing IE, for example, can be done using a virtual machine with Parallels or VMWare Fusion). I definitely like that I can test all major browsers on both mac and PC from one machine. There is some software that I love that is Mac only, and that I can't really see living without... Coda, Textmate, Screenflow, Things, Moneywell, etc.