Any laptop will work. RAM is probably the first thing that will give you trouble over time, so don't skimp on that. More important that what computer you get is what OS and software you use. If you end up doing anything that requires computer models, you will probably be using a UNIX/Linux server. For this it is easiest if you are running Linux or Mac OS on your computer, but there are programs that will allow a Windows OS to run a bash shell (my friend uses putty I think?).
My friends that are doing more experimental work often use Windows. I think most experimental software runs on Linux and Mac as well, but their prof. uses Windows. I can only speak from my experience, but most of my professors were Linux people until the university bought them Apples!
I think Pengwuino's point is that any computer that goes online will do what you need it to do. Until this year, 75% of what I did on my computer was read articles online and look up terms that I didn't understand from my textbooks. The other 25% of my school computer use was using a graphing program to see complicated functions, using a spreadsheet, and [itex]\LaTeX[/itex].