Are Evesham still trading, I thought they had recently gone bankrupt ?
I would definately go with a Core 2 Duo (make sure they use the laptop varient of the chip). Pretty much any graphics chipset will do unless you intend to play lots od 3D games in which case 2 graphics cards will certainly help. You really want as much memory with the graphics cards as possible, the same with the RAM and also as large as possible hard drive as you can afford.
Keep in mind the screen type and resolution as there is no point paying for a super duper graphics card when your screen has a lowish resolution.
You mention that it will be a desktop replacement so i'm assuming that size is not an issue but weight may be. A 17+ inch pc on your lap is no fun after half an hour believe me.
Had you thought about an Apple ?, they can run Windows as well now days so you could get the best of both worlds.