i would only add that even with a dated old 450mhz celeron processor (back in 2000) i was able to use multitrack recording, for four tracks.
with my new laptop, i can run easily sonar for multitrack recording and use a good amount of vsts or other softsynths without having problems at all.
and i should mention that i use the same laptop for music, but also for eveything else that can be run on a computer: gaming (quite demanding of course), word processing, internet, music listening, video capture and editing, graphic and movies, webpage design, etc.
nowadays, the computers are very powerful, and even a 1000 dollars laptop is amazingly able to get the job done, when talking about making music.
but you need a good external (usb2 or firewire) audiocard.