Heh well a little late, but for anyone else curious...
Firewire Hard drives can be good for audio storage if they are the only thing on the chain. FireWire 800 is particularly decent. Now they wont be as good as say a RAID array by any stretch of the imagination, but they can still hold their own for most smaller projects.
You CAN replace the HD in a laptop if you have at least a small clue of what you are doing aroud electronics and feel comfortable doing. The Hitachi 7200 RPM HD is well worth the money, and I believ Seagate has one out as well that is also a good bet for the money. Be aware there are 2.5" HDs and your internal probably has an adapter to convert its IDE to whatever port your laptop has built into it. Do a google search, there should be plenty of info out there on this, but for audio work get a 7200 RPM.
Now on to the other things. Recently I was in a similar boat, but after much reasearch...
I decided to get a top of the line laptop wasnt worth it. I could do decent small project recording(All the MBox can support anyways if I remember right) just fine on a more portable processor such as the pentium M. In fact I plan to purchase a turion for just that purpose in a bit when they come out. I have my desktop for my more serious work. Dont think itll be a big deal though.
In as far as sequencers on the PC, a lot of good ones were already mentioned though Logic is no longer supported on the PC thanks to apple buying it from Emagic, or buying out Emagic, one of the two, cant remember which. As was Already said Nuendo and Cubase are both alternatives, but there are many others. But I believe ProTools runs fine on most PCs, but I have only used it on a Mac myself, even though I dont own a Mac.
Also I will put in a shamless plug or two for other alternatives, for those on a low budget, and not afraid to learn new things in as far as computers, get the Fervent Studio to GO or Download either the CCRMA or Demudi Distros of Linux and try out one of three things there, Muse, RoseGarden4, or Ardour(My personal Favorite) Ardour when it comes to straight audio already gives ProTools a serious run for its money and isnt even in its 1.0 yet. But it doesnt support anything else yet, no video no Midi. For Midi sequencing RoseGarden however takes the cake from what I have seen. Very nice peice of software and if they hurry up and put in automation on the audio tracks I would jump on it. Muse seems to be somewhere between the two in my opinion, doing both as well as the other, but overall it seems to be running just a little behind the other two.
Ok enough of the shamless linux plugs
Have fun with your new laptop.