Depends what you are playing. If the PC you are using has these specs, http://www.cnet.com/products/dell-optiplex-780-core-2-duo-e8400-3-ghz-series/specs/, then it will bottleneck in games utilizing more than 2 cores - like most MMORPGs for example.
Which games?
Here is a video of Battlefield 4 being played on the same CPU with an r7 260x (https://youtu.be/UtIHp6aOOno?t=40s), which performs slightly worse than the GTX 750, @ 720p low settings. The other thing to consider is the CPU may bottleneck the 750.