i forgot you`ll also need to be very up on processers & processer speeds to get the right one but from what i`ve seen out there you`ll need at least or preferably a 3GB clock speed. & right now on laptops thats bloody expensive
Dont fall for the whole "ghz" thing cos its balls. With Intels newest Chips the fastest one is 2.93 Ghz but loads more powerful than the old 3.8ghz ones. Although JT is right, you will need a decent processor, especially if you want amp modelling, otherwise there will be a noticable gap between strumming a chord and hearing it out the speakers.
+1 on the Point on Desktops unless you Really need the portability. Even if that is important you'd be better off getting a small Desktop. £900 will get you an upper mid range PC but an average laptop. also remember that if you want to upgrade to a faster processor, a bigger screen, a larger hard drive and a better sound card on a laptop, forget it.
Also remember that soundcards use power quick, so you wont be able to do long periods of recording and editing on a laptop without mains power.
And Finally (I'm Nearly Done now) If You do get a desktop I highly recommend building it yourself. If I had bought my PC from a Big company, it would have been £1k+, As i built it myself it only cost £600. It really isn't very difficult, just set aside half a day to build it, I have had no problems with my PC that I built myself with no previous experience. If you Buy a high end PC from Dell etc. You'll probably end up with a gaming PC- So you'll end up shelling out hundreds for a graphics card you wont use. I can however, recommend that £800 Macbook pro if you really need a laptop. And dont Jump the gun before you choose what to get if you get a windows based machine. In the coming months, AMD will release their new Processors, there'll be DirectX 10, Windows Vista, so half your stuff will be obsolete pretty quick.