Newegg is having a sale today on a quad-core 15.6" notebook with dedicated graphics.
here it is.
Here is the information on the graphics card. This site is fantastic for referencing what your mobile graphics card can do. I'd give it a check before any purchase you think you'd be doing gaming on.
Finding a quad-core in the sub-$800-900 range limits you to AMD, and on mobile I don't think a quad-core is necessary. The only way you'd get a quad-core by going Intel is their older Core2Quad line, as only the i7 is quad-core on mobile -- and that's only select models. You'd likely be much better going with an i3 or especially an i5 with hyperthreading + a better graphics card than to spend more money on quad-core.
As for the remark of "only" 512mb of graphics memory, you'd only notice that if you are loading large amounts of textures into the card, something I kind of doubt you'll be doing on a mobile part. You'll need to play at a lower resolution so you can get a playable framerate, meaning you'll not be using as much of the bandwidth or actual memory of the card as you'd think. Even with games I play on my laptop with a 256MB video card, I can see the memory get filled 80-90% and be fine. This is also with a dual-core processor, non-hyperthreaded. That being said, I'd suggest the 512mb as the lower end of the spectrum when thinking graphics card memory.