I have an ancient G4/400, my basic setup is 32 tracks. I have tested up to 50tracks with Cubase5, and DP3.
I own Pc & Mac, the biggest difference is the user experience, less research is needed for the Mac. Any hardware that I have
added allways works the first time.
PCs are cheaper, but usually buying off the shelf brings the pain of incompatibility. It takes more time to research hardware, many argue that the info is free. but if you are a business it is time spent not billing a client. (at $40-$60 per hour it can add up quick)
Pc's are faster currently, and the Mac world is in a transition to OSX, that will probably take six months to sort out. (there will be some pain with the transition until all drivers, and plugins are upgraded.) Currently all of the large sequencer companies have commited to OSX, so now it's a wait and see.
PC's are cheap, I think both platforms work well together.
In my business I have to interface Pro Tools and Avid systems, most of these are Mac-based. Macs dominate in the pro media creation field.
At the present time it's hard to compare the speed of Pc's to Macs.
generally speaking if you add 200Mhz to speed of a mac you will get the equivilent PC. ie. 800 MHz G4= 1Ghz PIII.
You can jump to a Mac now and be productive under OS9, and make the transition currently everything works.
I'm not swithching to OSX untill everything is in place.