Three things.
1. Buying second-hand can really stretch your budget. My dad found my Acer Aspire 1410 11" for some ridiculously low price, 6mths old, and it was used about four and a half times by the seller's teenage daughter. It has a proper 1366x768 display and a Celeron dual-core processor, so it completely covers every base I need from it.
2. If you want to take notes in class, I suggest a small laptop (10-13"). Portability is a big factor. Beware of true netbooks, though, as the lack of computing power and low-res screen may become very frustrating.
Many people will tell you at this point to buy a tablet with a keyboard extension; I disagree as tablets have much reduced functionality (can't install proper programs on them).
3. Depending on what you're studying, battery life is something to take into account. I have long hours of lectures and tutorials every day, so an i3/i5 would probably not last long enough. If you only have a few lectures a day that's obviously not an issue.
At the top of my wishlist is a detachable laptop (e.g.
http://www.asus.com/vivo/en/transformerBook.htm). This is probably the ultimate student computer, but it'll be a while before one of those will fit into your (or, for that matter, my) budget
