To Add to this: When I started out I did the mistake of going the laptop route. If you plan on doing 3d modeling and graphic post-production (even CAD), I would strongly discourage anyone from getting laptops. I built a tower in school and still use it this day. When I was in studio all my classmates used expensive and newest $$mac books$$ only to see their 12 layer Photoshop drawings come to a screeching halt and laptops heat up to lava. Meanwhile, my tower was running complex Rhino models without even a hint of issues while running AutoCAD, Photoshop, InDesign and GIS all open at the same time. Whenever my classmates walked by they were perplexed as to how I had so many programs running at the same time.
Laptops are not designed to handle heavy processing (unless you go the alienware route and are made of money). If you want to make an investment that pays off well and can handle upgrades over time, towers are your best bet.