Fanum is spot on.
Are you a corridor cruiser, someone looking for a powerful desktop replacement whose portability requirements are limited to being able to move it around the house and carry it in a car from home to work? In this case weight is unimportant but you'll probably want a big 17-in high-resolution screen with powerful graphics, and you won't be too interested in battery runtime. Many of these systems have high quality audio speakers and some are designed for graphics-intensive gaming.
If so (using Dell as the example), look at the XPS M1710 (from £879) for gaming or corridor cruiser needs, or Studio 17 (from £429) for just the large screen element.
The other extreme is the road warrior, the user who always carries their laptop with them who wants light weight, small size and extended battery life. A 12-in or 13-in screen is normal here, though there are smaller alternatives. Speakers are likely to be low quality. You may decide you don't need a DVD writer, just a DVD reader, and maybe don't even need a CD drive.
For a system with 13-in screen look at Dell's M1330 (from £629). For ultra small and light look at Dell's Inspiron Mini 9 (from £299) which has a 9-in 1024 x 600 screen, up to 16GB solid state drive and weighs just over 1kg. You might want to budget some additional funds for a CD/DVD drive which would connect to one of the USB 2.0 ports, plus a wifi card. The really nice thing about Dell's Mini 9 is that it comes with Windows XP rather than Vista!
From what you've written you're neither of those two extremes and are a middle of the road user. There's a wealth of different notebooks out there, most of which come with a 15-in 1280 x 800 screen (because that's the cheapest to manufacture). Almost all of them have perfectly adequate processors, enough memory and hard disk space, and will have built-in optical (DVD or CD) drives and both RJ45 LAN connections and wifi. Dell's Inspiron 1525 (from £329) or Studio 15 are examples.
I have two systems, an aging Dell corridor cruiser with a 15.5-in screen with a wonderful 1920 x 1600 resolution, and a PC World Advent road warrior with 12-in screen (1280 x 800), DVD writer, firewire, etc. I take the Advent with me on the road and when I want to use an Internet cafe just unplug their desktop and plug in my notebook. The battery runtime on the Advent is the weak point but I have a 12v charger which means I can top it up on the bike.
Tim