One thing to keep in mind when getting a laptop is to go for high resolution (not necessarily screen size; make sure you understand the difference).
One of the reasons working on a desktop for music is more fun than a laptop is because of the available screen size, it's nice to be able to put your arrange window, your mixer, and some instruments or two on the screen at the same time. So do yourself a favour and get something with a really good resolution (IMO 1440x900 should be the absolute MINIMUM, any smaller and you actually will regret getting a laptop).
Also, get warranty coverage for your laptop. I probably shouldn't have to tell you that but some people are cheap. If you're going to put down a few grand on a computer that you're going to move nine times a day, get at least 3 years coverage on it because it's not an issue of if it breaks, but when. Laptops are fragile, you can get the best consumer-grade laptop around and treat it like fine china. Something's going to break on it.
If you are thinking of getting a USB controller (IMO again, a controller is worth it, they make music more fun), there are some ultra-portable ones out there like the Korg Nano series that are super compact.
And YES do be aware of what sound card is in it. Try to look up the chipset for it and doublecheck that the drivers don't suck. More than likely you'll want to check if there's good ASIO drivers for the chipset.