my two bits
I had to answer a similar question a little while back (i.e., what computer to buy) and decided on a desktop PC, purchased from a company where I could specify the components, for the following reasons:
1. An experienced sound engineer strongly recommended I purchase a computer with an Intel Pentium chip, not an Athalon or whatever, because it was more likely to work better with audio software (multitrack recording or sequencing programs, etc.).
2. Desktop over laptop because I was buying a machine primarily dedicated to recording (which I think is also good advice) which for me didn't require travelling and the desktop seemed easier to upgrade myself.
3. I went with this particular company because it allowed me to specify the components I wanted at a better price than buying a premade system and upgrading a couple of things. However, I've since seen cheaper prebuilt systems with all the components I need that I would purchase instead.
Regardless of the above, it seems reasonable to figure out exactly what your entire setup will be now so you can plan for the chipset, soundcard, RAM, etc. that will meet your needs down the road.