I'm a recent switcher who traded his PIII 850 MHz Dell laptop for a 550 MHz TiBook back in January. In my honest opinion, you will probably notice a significant speed decrease if you are running IE or Word on OS X. Jaguar has improved OS X's performance, but it still lags behind a P III. Webpages load much faster on a PC, although as other reviewers have pointed out, it may have more to do with the browsers. If you run it on OS 9, however, web surfing and processing is pretty fast.
Nevertheless, I have few regrets in switching. My TiBook has handled many tasks for me without greeting me with a blue screen of death. You can plug in any hardware with full confidence that it will work on a Mac (I was reminded of this when I had spent a frustrating hour trying to get my PC at work to read the external zip drive). PC users have come to expect crashes as much as humans expect the rain--a natural occurrence. Rather than "Think Different," I think the new Apple slogan should be something like "Mac: It just works."
Here are some of the programs I run: Office X, AOL IM, Mozilla, IE, REALbasic, iTunes, iPhoto, and FileMaker Pro. I have Photoshop 7 but I can't compare the speed between P7 on a Mac and PC since I had never used it on a PC. It runs fast enough for my purposes.
In my direct experience, I have found PCs to be faster than Macs, but far less stable and compatible with 3rd party hardware.