You have to make a decision between being able to play games, and having a battery life measured in hours.
Laptops with a discrete graphics card (by the way you can't upgrade graphics cards yourself in a laptop) will be expensive, heavy and may have battery lives of half an hour when playing. Laptops without will be light, portable and less expensive.
SC2 and Portal 2 will likely need a discrete. If you do go for discrete it is important to get AMD Mobility Radeon 36xx and up/46xx and up/56xx and up or Nvidia Geforce 9600 and up / GT 130 and up / GT 220 and up / GT 315 and up / GT 420 and up. That's all the cards with 320 and 48 shaders respectively.
Have you considered buying a desktop (they are much cheaper for what you get) for gaming, and an ultraportable for the battery life? It would come to a similar total to a gaming laptop but you get both aspects and also the desktop will handle any amount of new games.
