It won't be easy finding a gaming laptop for the price range of $600. This is because i7 laptops with good gpu usually costs $700+, and laptops below $600 with good gpu are i5's which is a dual core on mobile platform. So i3's and i5's are pretty much the same on mobile platform, minus the turbo boost.
It has been reported that quad core is current sweet spot for gaming, and multithreading is the future. So I would recommend that you build a desktop pc around AMD FX6300 cpu or FX8320 + HD7790 gpu or higher and try to work the budget around your goal for other components.. The highest gpu you can go within this budget the better. If you want to go Intel, try 3350P or 3470, depending on your budget. For long term planning, you can go 3570K, but you will probably need to go with lower end gpu now, and upgrade later (which defeats the purpose of gaming pc)