Anymore it just doesn't matter.
I made the switch to a laptop a couple of years ago and have zero problems.
Realize that you're going to need an external drive or two (one for recording projects, one for sample librarys.... maybe another one for backups. But they are cheap @ $100 for a 500-750Gb usb or firewire drive now.) In a desktop, you'd similarly want 2 or 3 drives to split up the audio functions.
Firewire is a much more robust data channel than USB - you can work with 100 audio streams instead of a handfull that usb2 will suport as firewire was designed from the ground up for SUSTAINED high-speed data transfer. Just realize that you will NEED a TexasInstruments firewire chip either in the motherboard or add-on card.
I have a single-core 2.8Ghz P4 laptop (with TI firewire chips) running to a Motu828mkII daisy-chained to a Glyph firewire drive, plus I use a usb2 freeagent drive for sample libraries and video editing. ZERO problems and I've worked with up to 35 tracks. A dual-core laptop will give you better performance than mine.