Honestly, if you're thinking long-term you're better off with one of the cheaper i5's. There is not a huge price difference. That way you're set up for future games, that will likely require a quad-core processor. Some current games even require a quad-core. It's cheaper to upgrade a GPU than a processor down the road because the sockets change so often that you normally need to buy a new motherboard and maybe even RAM every time you upgrade your processor.
But if you already have the i3 there is no reason to upgrade it yet.