A good rule seems to be spend the same ammount of money on both. Let's say you got a 100$ CPU then you should get a GPU around the same price.
Then it kind of depends on what you are running. BF3 likes multi cored CPU's so probably an i7 would be better then an i5
Wait until june/july until kepler, and intel ivybridge. This is another reason why I am waiting too, both kepler and ivybridge bring more performance and use less battery life. This means your gaming laptop will have roughly 30-50% more battery life probably bringing it around 3-5hours of battery life instead of 1-2.