My advice would be to go with a good 2 channel set up. A real sub in a dorm rooms is ridiculous because you'd never be able to use it because you'd instantly get reported to the RA every time you did.
Go with 2 good quality bookshelf speakers and a good 2 channel receiver/amp. If you intend to expand it later into a surround system (when you move out of your dorm) then buy a multi channel receiver.
Trust me, anything beyond 2 good bookshelf speakers is overkill in a dorm.
My bedroom set up is two Klipsch RB 61s hooked up to a Pioneer Elite VSX-21TXH. They won't go as low as a sub will, but they still deliver great sound in a small room. You would be amazed at how good a real set of bookshelf speakers sound in a small room.
If this is the direction you'd like to go, I can recommend some bookshelf speakers that I think would fit your budget. My preference was Klipsch, but you need to listen to a bunch of speakers before you know what brand you want to go with, so I would prefer to provide you with options rather than just recommend what I use. What I like may not be what you like.
Otherwise, if I still haven't convinced you that you don't need a sub, then buy a sub from one of these three companies:
http://www.hsustore.com/http://www.svsound.com/subwoofershttp://www.outlawaudio.com/products/index.htmlThose 3 companies are basically factory direct audio companies that sell their products directly to consumers via their webpages. They make very high quality, reasonably priced audio components specializing in subwoofers. They are by far aways your best bet for high value subs. If you do your research and look through any of the hifi webpages, you'll see that all three companies are heavily recommended by people who are obsessed with this shit. Their subs are often compared to "name brand" subs that are often 2-3x the price.