I bought a new PC to run games like those, the insides can be interchanged not like a lap top. I had the same problem with my brand new Dell laptop, video card wasn't strong enough.
You can buy external video cards for laptops but you would also need an extra power source to stop the battery from dying so fast.
HP has some decent desk tops on sale which can run Diablo 3 and Starcraft 2 at the low setting. Then you can purchase separate video cards, ram sticks, etc to make it run better.
It's hard to keep up with technology as every year the new stuff you buy is constantly being outdated for something better.
My best advice is to get a desk top for gaming. Or spend 3000 dollars on a nice alienware laptop if you're looking for a portable computer system.
Also keep your eyes out in the future for the FIONA- it is made by razer and is a portable gaming computer with all that shazam built in. http://www.razerzone.com/projectfiona