I use a PC for my music production, but I've got a macbook for home a/v duties...
Seeing as the intel platform is becoming a "standard" for both OSes, its a fairly safe bet that like-for-like comparisons are easier to accomplish now..
I've got a 2.6GHz Core2-based 2GB RAM Windows XP PC that I use for music, running Cubase SX.. a whole load of Virtual instruments including some rather "system intensive" ones like Spectrasonics Atmosphere, Trilogy and Stylus RMX.. It doesn't even break a sweat when running multiple tracks (say on average 15-20+) of that, audio with a bunch of plugins, eq etc. (less than 20% CPU utilisation at a guess). On a machine of that spec, you can also expect things like rending an audio project of similar complexity, to a Stereo 44.1KHz WAv file to take about 40secs
With that in mind, hopefully you'll have enough info to scale up/down your requirements as necessary.. I *used* to do my production on an Athlon XP 2100+ which was getting on a bit hence the upgrade..
The main thing whatever you guys choose to go with, is that you spec up something with at least 1GB Ram (minimum), 5400RPM or better HDD and an audio solution with good solid low-latency drivers.
Installed system RAM is important, but so is the HDD you choose to install (when using instruments that have a lot of sample data on disc.. eg. Atmosphere, The Grand, Gary Garritan strings etc). Also make sure that you don't have an installation of your OS with lots of rubbish installed that may eat CPU cycles in the background without you realising it (bear in mind I'm talking from a Windows perspective here but the same is kinda true with OS X too)

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