I look at it two ways and ask the question, "What makes a Mac a Mac and why can't I call a PC a Mac too?"
You can run Windows XP or VISTA on a Mac and Mac OS X on a Mac at native speeds without the need for virtualization but rather under the Apple branded tool, BootCamp. I can't necessarily do that on a 'PC'. While a lot of attempts to make OS X run on a generic box have been made, only a limited handful have been successful. I'll look at why that is in a moment.
If you're anything like me you need the computer you have to remain as useful as possible for as long as possible. Macs tend to retain their value for an average of five years and a tricked out PC an average of 3. Price isn't so much an issue these days as the Apple move to Intel chips in their hardware has significantly lowered the price of their computers. So you CAN replace your PC with a shiny new iMac or Mac Pro tower. I'm into portables so, replace the old space hogging clunker with a new Macbook Pro. Load up your favorite version of Windows and also keep Mac OS X on the hard drive and be happy.
So if you were to go pound for pound on a PC portable with a Macbook Pro (and don't skimp) you'll find that a PC (any brand) will quickly run about the same or more than the Mac. Plus with the Mac you get two computers in one. You can't buy two equally powerful computers for the price of one- and you wouldn't want to carry them both around with you either.
So why can't you run OS X on just any brand of PC or compilation of parts? Well remember what makes your PC useable to you is an OEM OS packed with custom drivers for every piece of hardware you have installed. Now just try to find the drivers that will run under Mac OS X. If you're a programmer you might be able to get something going but those projects are limited to running ancient versions of OS X. There is little to any hope that the hardware will continue to work as expected after the next OS X update. That's the hardware limitation- the legal reasons is that Apple refuses to license OS X in any form to anyone else. If Microsoft decided to take this approach they'd have to begin producing their own hardware and trash their entire licensing based business model- aka, ain't gonna happen.
Don't switch to a Mac and feel locked up and torn away from all you love and know about your PC. Instead think of it as embracing the value of a true two computers for the price of one scenario and continue using Windows AND Mac OS X on your new computer. 