I agree with Urza (not the first post, but the last one). Anyway, since I've answered this a tonne of times already, I don't see the harm in doing it once more.
Laptops have a scale these days. On one end, you get the ubar1337 gaming laptops (read, alienware), and on the other size you have the flimsy netbooks that last 12 hours (reportedly). Gaming laptops are lucky if they last an entire hour (unless you get one with IGP and discreet graphics that can switch between them). If you want to play games, you will not get a good battery life. Unless you intend to spend a fortune on chemical batteries (they last a day or 3, but cannot recharge).
So let's say on one end, you get the Alienware M15x. It'll handle any games you like but have crap all battery (40m I believe). On the other end you get an Asus CULV that lasts 8+ hours, but it only comes with a single or dual core 1.3Ghz CPU and some kind of crappy IGP. You won't be playing games on it.
In the middle, there's the Alienware M11x that just featured in the CustomPC magazine I got a couple days ago. It uses an overclocked CULV CPU at 1.6-1.7Ghz (with options to get more powerful CPUs, at the cost of some battery life). It's a good balance between power and performance and can play games at a decent quality (even if the screen itself kinda sucks, with very limited viewing angles), but has a small size. 11.6" is actually good for taking to school since it's small and portable, but personally I would never use it to play games unless every other PC/laptop in my house is already occupied. Anything below 15" to me is too small for gaming.
Another alternative is to take the same path as my dad - the Acer Aspire 5740. It's a 15" laptop with a Core i5 and i5's own integrated GMA HD4500. The IGP sucks as all IGP suck, but it's good enough to play Borderlands and Left 4 Dead, so it'll be fine with Guild Wars. It also has a 4.5h battery life, which is pretty decent considering that it has a 2.26Ghz 2+2core CPU. It only cost him £550 too. There are other laptops too with the same idea - if you just stick with a powerful Core i5 and its IGP, then you don't have to spend energy on a discreet GPU so the battery lasts twice as long. This makes it an ideal balance between power and battery life, which sounds like exactly what you want.
Or you can wait a year for the Sandy Bridge CPU to come out, but it sounds like you don't want to wait.