I bought a laptop when I was in a dorm, and when i first moved to an apartment because i didnt have room for anything else. It was nice for a while, but I am glad to have a desktop. It's nice to sit back and use the computer, instead of being up close all the time. Unless you want the portability, and don't play a ton of games, keep or upgrade your desktop.
You also probably aren't going to get much out of your desktop anyways. Desktop parts are so cheap, people figure they might as well buy new. You may find a sucker to pay a lot for your computer, but with nwe full computers selling for $299 you're not going to have much luck. I'd personally just get the laptop and keep both of them.