Let me preface this by saying, I have a PC and Mac, and think they're both good for what they do.
I have a 667 MHz G4 Titanium Powerbook...and it's the $*^t. It's got a built in Firewire port, 2 USB ports, DVI port, SVIDEO out, as well as a fast CDRW drive, and Gigabit Ethernet. Used, you can pick a similer powerbook up for around $1200 - $1300...and it's well worth the money. These things are rock solid, and most of the applications that run on Mac are also EXTREMELY stable. Don't compare the clock speed on PC's and MAC's, they just don't translate into numbers that mean anything at all. It's like compairing apples to monkeys. Let's put it this way, my 667 MHz G4 handles most things as quickly or better than my 1.4 GHz Pentium 4. That's just the truth of it. They do not compair well, they are totally different processors. It's kind of like compairing a Pentium 2 and Pentium 4 of the same clock speed...it's a number that is based on a set of instructions...so the set of instructions have almost a bigger impact on the real speed of the computer.
I have a MOTU 828, which runs on firewire...and sounds great. In my opinion it's one of the best boxes you can buy under a grand...and used comes in around $550. It also contains two Mackie style preamps...which are good for location work, when you don't want to bring along a huge rig. I'm running Digital Performer, which is a nice audio program. If you're looking for ease of use, and most compatiblity, you should check out the Digidesign Digi002 rack version. It's got 24 bit 96KHz converters, and comes with ProTools LE. This program is the easiest, most stable audio program there is. Especially if you do just audio, and no MIDI. It can handle MIDI, but if you want to do both, Digital Performer works better in my opinion.
I'm sure there are some good things going on over in the PC world, I've just not had good experiences with AUDIO over there. Then again, it's been a couple years since I used my PC for anything but business software and games...which it's definately better at than the Mac. The mac definately can handle the business stuff just fine, but there's more out there for the PC. The PC is also way better if you're going for gaming stuff...the Mac selection and performance just sucks there. If you're going for audio, I'd say that the Mac handily beats PC's, expecially in the Laptop arena. The built in firewire is a huge plus, and goes a long way.
Hope that helps.
Jay