The HD 6790 is the faster card while the HD 7750 is newer and more power efficient.
Also i've got no idea how much you can spend on a processor but im pretty sure you can afford a AMD Phenom II X4 965.
If you can just buy it.
I would grab the i3, it performs very similarly to the phenom and being able to upgrade to an i5 later if needed. If you wanted to upgrade from the phenom you would have to hope that piledriver/trinity/etc would be able to get it or switch to something from intel which includes the cost of a proc + mobo.
Just food for thought. Haven't looked at the prices though.