Also if you look at Dell, for £900, you'll get a decent processor, 512mb Ram, and a low-end graphics card, in a laptop.
In a desktop, expect a much-faster processor, double the Ram, and a mid-range graphics card.
The added cost of a laptop is that everything has to be made portable and power-efficiant.
Also don't bother with a gaming laptop if you intend to take it into school/college. Oh how we laugh when those who bought alienware's for Uni realise carrying 5 kilo's of Nv 7800's around is a bit unnecessary