In your case, I'd be looking to move up to a Core i5 rather than a new GPU. You've saddled a pretty top of the line GPU with a rather low end CPU, so if your motherboard will take an i5, that's what I'd suggest doing, and then maybe another 8GB of RAM if you have any money left over.
I'm hoping to get a new graphics card soon. The one I have at the moment is a 2GB AMD Radeon HD 7770. Runs everything almost at optimal level but it's not quite as smooth as i'd like it to be in some areas. Just looking for something within the £200-300 price mark.
Any advice would be greatly appreciated as i'm not the most knowledgeable of people when it comes to this sort of thing.
Intel Core i3 2120
Let me know if you need any more info