X51 R3 Graphics Card Upgrade

Hi all, I've had my X51 R3 for over a year now and I currently have an Intel i5-6400 @ 2.70 GHz and the NVIDIA GeForce GTX 745. I also have the 330w PSU. I've done some research but I am still a little lost because of how many different models of the GTX 960 there are and which ones can fit in the X51 R3. The main purpose for this PC is for gaming and I feel like I can't play some of the games I want to because of the GTX 745.

I am looking to upgrade the GTX 745 to a GTX 960 or maybe something better depending on how much I decide on spending. I know next to nothing when it comes to the inner workings of a PC so I need a little help on what cards I should be looking at.

My other question is, is the i5 I currently have good enough, will it work with an upgraded graphics card or will I need to upgrade that as well?

Thanks for reading, and I appreciate any help you can provide.