Upgrading X51 R2 help

Hi, first post here, apologies if I'm not on point...  windows 8.1  64 bit

I have the following specs on my x51 : i3 -4130  @ 3.40GHz 

graphics card GTX 745 with 4G memory

RAM came stock wtih 8GB  

I wanted a little more juice so I just recently upgraded ram with 16GB kingston.  Very nice noticeable performance increase.  Much snappier in general and can run games on higher settings.

I wanted advice on what to upgrade next.  My goal is to play witcher 3 and similar games with maxed out settings, or as close to it as I can.  Currently I can play with  framerates at 30 fps,  at maxed resolution (use a 50in LG LED for display)  but I can't push any other settings too much, its running on medium with a few tweaks etc.

I'm no computer whiz, but I know the basics and the mechanical sides of installing components.  Now I think upgrading the chip would be my next priority.  The i3 is not ideal, but I'm wondering if upgrading card would do more.   Kinda going for biggest bang for buck.

The GTX 745 while hardly top of the line is still a decent card from what I understand, the 4 g Ram on the card help a bit I imagine, so I suspect my weakest link is the chip.   But i'd really like to hear from someone more knowledgable (smarter :) ) than myself.  Going to i5 seems like a no brainer.  For a primary gaming machine the i7 from what i understand wouldn't do a whole lot more than the i5 and wouldn't be worth the additional cost.

Any and all suggestions appreciated.