As fun and easy as a laptop sounds, I'd highly recommend getting a normal desktop. A desktop for the same money will have a lot better stats. Will you be recording a lot on the move? Also it's a lot easier to upgrade specific component in a desktop than in a laptop. Another point is monitors. When I am producing I use 2 or 3 (24") monitors, just go have an efficient workflow. On a laptop this will be more difficult, since laptop monitors are small (if they have big monitors they are usually heavy as hell, and the standard max is around 19" I think) and it works better to have the monitors in the same size.
Basic guitar software will be no problem with a normal laptop, but if you want to get into serious recording/producing, you will need some serious PC power. 1 track is not a problem, but when you have 10+ tracks going at the same time, it will be heavy for your PC (especially if you use VSTs). I have a quad core with 4GB and still it doesn't run as good as I'd want it to. 4GB ram (or 3.2/3.4 I think) is the max used by 32bit Operating Systems.
You will need a new soundcard regardless. I have yet to see a stock PC with a soundcard that is acceptable for good studio work. I have a firewire audio interface and I think most people do (you do have some PCI-E interfaces, but that will do you no good with a laptop). IF you decide on a laptop, be sure it has firewire ports. A good soundcard will set you back AT LEAST $150-200. Some will say 'yeah, well my $50 Creative works fine!', but believe me a good soundcard is essential to any digital studio. A good entry level 'pro' card is the t.c. electronics desktop connect 6 (firewire).
MAC is pretty much the standard in the professional industry, though Windows will work as well. It also depends on which software package you intend on using (for example FL Studio isn't available for MAC). Some software packages also run more efficiently on a 2 core processor (or not significantly better on a more expensive 4 core), so you'd need to make sure the PC you purchase is optimal for your needs.
Crux: will you be moving your PC around? If not, get a desktop. Also look into which program you will be using and which PC/OS is best for it. And get a good soundcard.
Edit: just noticed your headphones comment. Grado is a good brand. But, if you want to produce properly you will need studio speakers. I've done many tests in this field, since I wanted to solely use headphones at first, but headphones colour the sound. Since you're recording guitar I presume they are closed cans, but that also means you lose the eagle eye view of the sound (the sound waves are confined/obstructed by the casing obviously) - (semi)-open ones are slightly better, but still not optimal. Speakers will put the whole thing into the right perspective. Don't get me wrong though, cans can work to do a degree. But to do proper recordings you will need studio speakers.
Sorry for the long monologue. I might come off as c--ky at some points, but I mean well. I went through the same dilemmas and did a lot of research into the matter :).
