In terms of overall gaming performance, the graphical capabilities of the Nvidia GeForce GTX 750 Ti are significantly better than the Nvidia GeForce GTX 650 2GB.
The GTX 650 has a 38 MHz higher core clock speed than the GTX 750 Ti, but the GTX 750 Ti has 8 more Texture Mapping Units than the GTX 650. As a result, the GTX 750 Ti exhibits a 6.9 GTexel/s better Texture Fill Rate than the GTX 650. This still holds weight but shader performance is generally more relevant, particularly since both of these GPUs support at least DirectX 10.
The GTX 650 has a 38 MHz higher core clock speed than the GTX 750 Ti and the same number of Render Output Units. This results in the GTX 650 providing 0.6 GPixel/s better pixeling performance. However, both GPUs support DirectX 9 or above, and pixeling performance is only really relevant when comparing older cards.
The GTX 750 Ti was released less than a year after the GTX 650, and so they are likely to have similar driver support for optimizing performance when running the latest games.
The GeForce GTX 650 2GB and the GeForce GTX 750 Ti have the same amount of video memory, but are likely to provide slightly different experiences when displaying game textures at high resolutions.
The GTX 750 Ti has 6.4 GB/sec greater memory bandwidth than the GTX 650, which means that the memory performance of the GTX 750 Ti is marginally better than the GTX 650.
The GeForce GTX 650 2GB has 384 Shader Processing Units and the GeForce GTX 750 Ti has 640. However, the actual shader performance of the GTX 650 is 406 and the actual shader performance of the GTX 750 Ti is 937. The GTX 750 Ti having 531 better shader performance and an altogether better performance when taking into account other relevant data means that the GTX 750 Ti delivers a noticeably smoother and more efficient experience when processing graphical data than the GTX 650.
The GeForce GTX 650 2GB requires 64 Watts to run and the GeForce GTX 750 Ti requires 60 Watts. We would recommend a PSU with at least 400 Watts for the GTX 650 and a PSU with at least 300 Watts for the GTX 750 Ti. The GTX 650 requires 4 Watts more than the GTX 750 Ti to run. The difference is not significant enough for the GTX 650 to have a noticeably larger impact on your yearly electricity bills than the GTX 750 Ti.