I was mainly using PCs for years and such and while I can handle them just fine. I ended up getting a macbook for the new job (php/mysql/css/etc developer, yada yada yada).
Now considering you're mostly familiar with microsoft technology (though you say moving to php/mysql) I would recommend you stick with a PC, even though you can load windows up on a mac (just seems like kind of a waste to use a machine that treats your primary OS as a supplement).
I'm pretty much running a small installation of XP on VmWare fusion whenever I need to test a site in IE7 or IE8Beta (or if theres any small application I can't run under OSX), and seems to have very little to no performance hit in the usual habit of having couple browsers open, dreamweaver, FTP application, terminal and so forth.
It's also nice cuz I recompiled apache (which isn't hard to do) as well as PHP5, and added MySQL5 replacing the default install the system had, so working off the machine locally is very easy for me, and quite fast.
But basically a little macbook, apple's cheapest, was rather painless for me (as a developer), and doing all my work off it even using the virtual machine (VmWare as it seems to work a heck of a lot better than Parrallels) was equally as painless. Quite easy to test against FireFox (Windows and OSX), Safari, and IE7/8, and booting up XP in the virtual machine only takes less than 15 to 20 seconds for me, theres really no need for Vista in my opinion.
If you do go the mac route, hit me up on some software recommendations (that are freeware/GPL) for the migration if you don't already have things like dreamweaver. and also about getting the default Apache install to have PHP/MySQL enabled (in a cleaner/native fashion than say LAMP/MAMP and such, and less likely to break as I've experienced).
Karl Blessing