What subject are you studying, and what are you thinking you'll be using the computer for?
If possible, wait as late as you can until you buy it -- if you only start in September, then try and get it late August.
Try and go for one with 1GB RAM, as much hard drive space as you can find, and you'll probably want wireless.
If you're just typing essays and browsing the net, then strongly consider getting a laptop. More expensive in most cases than a desktop, but a lot easier to take with you if you're going somewhere, even just home for the weekend so you can still do your homework.
Media edition comes with a little extra interface to the normal 'windows' look, that lets you do things like watch your DVDs and listen to music if you're sat away from the computer (turns your computer into more of a media centre).