Depends what you want it for - make a list so you know exactly what you need.
If you want to carry it about a lot, you need a small light one.
If that isn't important, get a big a screen as you can.
What do you intend doing with it? High end gaming or video editing needs a much more powerful computer than surfing the net and word processing, for example.
It is pointless paying �800 or more for a high spec machine when you don't use it to its full potential and a �300 one would do the job.