First of all, having a faster gpu will always give you a better gaming scenario. Having a weaker cpu wont make a better gpu play worse than a lessor gpu, if anything, youll see the same performance. Id do as stragestranger said, lower your res, see if your fps is higher the same. It should be higher, as the gpu has less work to do, less pixels to fill, and if your cpu is fast enough, itll show the gpu as working faster (more fps). If its the same , then its all your cpu can do.