Listen hanna thee best thing to do is to look at websites as gadget show and other mags and check out what they recommend.
I got a Sony Vaio NS series laptop 3gb ram 250gb hard drive for £430 off play.com and its amazing. How can anyone say sonys are bad? I know loads of people in high profile jobs who use the vaio. From my experience, Dell and HP are awful.
OP, do you have specific requirements about the size, weight of the laptop etc.? If it's something that isn't going to be moved too often then look at a 17" or even a 20". If it has to be quite portable, then 14" or 15" is best. If you always want to have it on you, then go for 12" or 13".
But remember different sizes cost different amounts of money. Extremely small and extremely large costing more.
y wudnt u just get a desktop? u gonna be playin much sims 3 on the go?