I've been using Mac for 15 years. Been working on Windows more than 6. Worked on Windows NT4, workstation, 98, Me (what a joke), 2000 and now XP.
Windows has very fast CPU, that's true. But they are also very noisy and that can be a problem in a studio.
Second, when CPU get hot in Laptop, they slow down. Unless you are actually using your laptop in a freezer (just joking).
Windows crashes (2000 does, and so does XP). I was at a Reaktor 4.0 demo yesterday, (West LA Music). The guy from NI had a PC. A Sony, 2 Ghz... Well, he crashed 3 times and had to reboot 3 times. He had sound distortion (he said he would probably need to reinstall his drivers). Good luck.
I also went to see the Apple event at the Groove (LA). The DJ had a Tibook and was running Live. I asked him if it was powerfull enough and he said yes. Of course if you run 200 VST plugin you might have a problem.
The fact is: yes windows is fast. But Mac can be fast if programs are well designed. A professional Musician told me that Logic runs better on a 450 Mhz Cube than on his 1.2 Mgz PC. Setting up Logic and all his stuff was painless. On Windows, good luck.
Conclusion
You want to go cheap and just use Live. Dude, get a Dell !
But if your sound card or other incompatibilities start crashing your computer, if you get viruses that erase or corrupt your files, if you need to install updates for security, then great.
Ableton is expecting to update Live very soon. Should support Altivec and Rewire.
And as for the OS, OS X is far more advanced than XP. XP is still windows. They just put another coat on it. And they're still fixing all their security holes.
If you also want to do Video, Photo, DVD, then I trully advise you a Mac.
Sorry, but PCs are way behind. And I almost forgot. Itunes 4 is also a good reason. check out
http://www.apple.com/music/ (will be available for windows later though).
IBM chip is coming. Hopefully it will solves all these issues and Ableton will have release a better and more optimized Live version.