DDR5 version is faster and better than DDR3.
The extra 1 gb of memory of DDR3 card won't come handy if you are not playing games at resolution higher than 1920*1080 i.e. full HD. And with 7750 its difficult to play games beyond that resolution without losing fps significantly.
So, better to choose a DDR5 version.
7750 is not good gaming card specially at that high resolution (1680*1050). you can try to overclock it but I don't think it would make any significant difference. Lowering the resolution might help though.
Better to go for a new card if you have the budget.