If you don't mind buying from Dell, they offer discounts for the poor college student (helped me with my purchase...). Your college should have a portion of the Dell site set aside.
Other than that, I'd say build it yourself, especially for desktops. You'll avoid some proprietary stuff that some places toss in to keep you locked into buying stuff from them. I like newegg.com for my part purchases.
Also, I don't remember any sites that offer services like Dell. The only other one I recall off the top of my is Alienware, but Dell bought them a long time ago.