http://www.gromacs.org/GPU_acceleration

If you check the page there, you get the best acceleration with the GPU. It's all there for you to absorb.

For not much you could try the usual Nvidia 750 as it's one of the best bang for the buck for apps like this. It also can avoid upgrading the power supply for now.

You would be spending a lot more and getting a lot less results if you go down the CPU only path.