Both will fit in a PCIe 2.0 slot (PCIe is backwards compatible).
If you're only using one monitor at 1080p or less, get the 750 as it has greater performance and you won't exceed the frame buffer.
If you have a higher resolution or more monitors, get the 650. Although it's performance is less, you'll need the extra frame buffer. (Something that will really slow you down if you run out.)