Nvidia just released their 7 series notebook GPU's. I wouldn't get anything with a 6 series unless it was STEEPLY discounted. The new models are are all significantly faster.
750 is around 20% faster than 650
760/765 is around 30-50% faster than 660
I haven't seen a 770-780 yet but the specs are good
Basically the new cards are up a step, the 750 is equal to 660 etc.
You can get the 750 SLI Lenovo for 1150$ here:
http://shop.lenovo.com/SEUILibrary/c...A012D9B20ED2FB
SLI 750m would be up to about 60% faster than the Asus G75 (with the 660m) depending on if you are using one or both GPUs. You also have little need for that much performance if all you do is LOL and WOW.
If you can get a good 15" 1080p laptop with a GTX765m for under 1,000$ that may be the best bet for what you do. Max speed would be less than SLI 750m but minimum speed would be higher. You might have to wait a week or two for all the new models to show up.