Just an FYI... If you upgrade the GPU to increase performance, the CPU load is likely going to increase with it. By how much is anyone's guess, but if you start increasing the in-game video settings, then the CPU will definitely need to work harder.

With that said, if you can afford the better GPU, then you should buy the better GPU, even if the cost-to-performance ratio isn't the best (which is usually always the case with top-tier hardware).