If you have a laptop or desktop at home which will be easily accessible, then a mini is great to take to class and write notes and work on a paper between classes, but you don't want to use it as your main computer.
Other than that, try getting one of the new Acers with the 10" screen (it won;t be so hard on your eyes) and make sure you upgrade the ram to 2GB, and if you think it will see a lot of bumping around, get a solid state hard drive, it'll take a lot of abuse and keep on ticking. Check out the keyboards on different models, they are not all created equal, some buttons are in different places, and some keys are slightly larger than others. You should be able to stay well within your budget of $400 too. Have fun with the little thing, its a fun toy and works great.