Wow, this is a big one!
First off if you're in audio production, be careful what system you buy. There are really bad latency issues with PCi-Express chipsets (as used by both P4 and AMD right now) and pro-audio soundcards.
The issue is caused by poor PCI implementation in PCI-Express chipsets, and nForce4 is the worst affected, Pentium 955X slightly less, and both seem to work better with low-power graphics cards for some reason. Anyway short story is to stay with AGP. You can read more about this issue on the RME website, as well as the SoundonSound forums.
Most people won't have a clue about this issue, but you should be aware before buying.
Then the question of what CPU.
The CPU is used when you're streaming tracks with effects on them, if you're not doing your submixes earlier. Obviously the more powerful the CPU, the higher your limits. A Sempron 3100 will do 10-12 simultaneous tracks without glitching (but at very high CPU usage, and you can forget real-time phase shifting if you need it), an X2 4400 can handle about three times that, plus two phase shifts.
Horses for courses. Obviously you have to work out what kind of a studio you're going for, then get the equipment according to that.
As for Pentium Vs. Athlon, it's basically up to you. the Intels run a lot hotter and require noisier cooling, but have a raw clockspeed advantage. The Athlons run cooler and generally perform better (assuming you're not having the issues with PCI-Express, which is just tragic as of now) but run at lower clockspeeds, ahich sometimes makes them a bit slower (for example, transcoding) at the same rating or pricepoint. The X2 4800 and FX60 will generally do whatever you want, but those cost too much money. An X2 4200 or 4600 is a good buy (the 4400 and 4800 basically are the same core with extra cache that will not help you too much), and will have enough horsepower to run a semi-professional project studio.
It's a misgiving that audio editing requires low power. It does not. I would run at 50% CPU running 8 tracks with 3-4 effects per track, and this was on an Athlon XP 2600+. With my new setup I'm not even trying, it's significantly slower (Sempron 2500/Socket 754). In the days of the sub-1 GHz CPUs, we would create submixes to get over the problems of low horsepower. Now effects are much more powerful, and very CPU-hungry. If you look at this
http://www.adkproaudio.com/benchmarks.cfm you will see how you can crush some very expensive processors with some audio and effects. 4 Opteron cores are what is required to handle some of the tests at decent latencies (anything less than 128 is what I would consider good latency).
I don't have experience with the business models other than the desktops at work, all of which are formatted and Ghosted with the company image before ever seeing a user.
A clean OS reload wasn't an option, this one shiped with an all-in-one restore disk. As far as I know it's still typical practice for consumer machines. Clean XP would have been an extra ~$2-300 added to the initial purchase. 
