It really does depend.
CPU and RAM are critical, but an often overlooked bottleneck is using the internal boot drive of the laptop,. It's best to go with 3.5" 7200RPM drives, preferably with a SATA interface and in a forced-ventilation enclosure with a 1394(firewire) connection. You also want to avoid "green" drives that will "sleep" on you, or at least drives that give you the option to turn that on or off via a drive bios tool.
Internal desktop drives tend to offer better price and performance over the 2.5" laptop drives. Laptop drives are getting cheaper though but tend to sit in the 5400rpm and slower speeds. Good for storage, but not ideal for production work. Even a DJ calling up audio, tha's sufficient enough production to demand the slight cost ugprade to the 7200rpm drives.
10,000RPM drives are somewhat hard to find via normal retail routes. Plus, they just tend to be loud. They remind me of tiny little jet engines. Expensive, durable, speedy, but not practical unless you're making lots of money. By expensive, they are a sizable cost over a 7200RPM drive.
As far as the Mac/PC debate, just look at this:
Running ProTools M-Powered, I use this on my MacBook Pro. it's a 17" model with the 2.66 Dual Core CPU(upgraded) adn 4 Gigs of RAM. Running as a PC, the Mac version smokes the PC version. IF you go with the "overpriced" MacBook, just consider you can run BootCamp and Windows OS's(XP and Vista, true Win7 support coming soon but it can run currently), you get 2 laptops in 1. Now it doens't seem so overpriced now, does it?
Also, since it seems everything Windows wants a piece of the action, it can be difficult to optimize a Windoze machine for best operation. You have some apps that will run when they want(like Norton AV for example) and doesn't come in and out cleanly.
But, in regards to interface, this is an often overlooked issue as well. You NEED an external interface to ensure a decent signal. I'll accept a LINE OUT, but I detest headphone outs. It's the wrong thing for the job. Internal line outs tend to be noisy, thus making a direct box still a good idea. If anything, you might need a ground lift, which would be on the direct box as well. Plus, I did an event at an air force base, and those are RF noisy environments, even leaking into the AC. I had to disconnect the power supply from the laptop to ensure a clean signal. I only needed to run 1 song off it. With RF bleeding into everything, fighting noise was a constant problem requiring lots of cable tricks and ferrite cores to choke it out to get things to what I felt was a good usable level.