Graphics card confusion

Solution
750ti. I've never heard of a 730ti, but assuming its a thing it would perform worse than a 750ti.

a 730ti would not be strong enough to really use 4gb of ram, so get the 750ti even though its only 2gb.
For future reference, Nvidia cards of recent generations tend to be named "GT" or "GTX" with the "GTX" being the gaming cards. So in many cases, regardless of vram amount, the GTX will be the better choice. Vram amount isn't everything, in fact it's often one of the least important concerns. More ram doesn't make a slow card into a faster card.