Cheap laptops can be had that are equipped with Nvidia 8600 class GPUs... we're talking $799 laptops with ample RAM. I really think something at that level of performance should be the minimum you're looking at. Anything less and you might as well just settle for playing Solitaire and Peggle.
I should point out that NewEgg's advanced search feature makes it easy to zero in on the perfect laptop. Just give it a price range that makes your budget and then pick the one with the highest class GPU. If all of the other specs on the system are up to snuff, you've found a winner.