My most servicable current combination for carrying around is a humble macbook. Its processor speed is 2.13ghz which is slightly slow but I've pushed it hard on occasions and its never crashed. I run Pro tools LE and Logic LE on it and also Final Cut LE.
I don't subscribe to Mac-Chic in any way shape or form and am currently having a custom PC built to spec for the current full version of pro tools which will replace a powermac.
If you're sticking with the PC road, I'd suggest using Windows XP Pro. Don't use Vista on any account!! I'd readily agree that its useless.

The Macbook still has a firewire 400 port which is utterly essential for transfer speeds with an external drive. If you use protools or logic, you have no choice but to use and external drive for your music data. Most other platforms will also perform better under these circumstances.

The current version of garage band is extremely useful as a music 'notebook' format but not a serious recording system. Don't be fooled by its ease of use!!.

Of course, if Logic as your weapon of choice, then A Mac is the only option.

For the best results in any format, a processor speed of 3 ghz is preferable.

Good luck.