What's your budget and do you plan on upgrading at a later time...(time-frame)?
What version of Windows are you currently running? I'll assume Win7 or later.
If the answer to the first question is no, I'd find a GTX-700 series card.
I currently run AC and RF2 on a GTX670 (slightly slower card)...and it is quite capable.
I have most settings at max and it is very smooth.
The GTX-970 is typically in the $325-365 range, so I wouldn't pay more than low 220-240 s for the 770 card.
If you do plan to upgrade, I'd go with the 970 right away.
The I7-920 is also quite capable of a good overclock with a good cooler.
Notice, I did not say an expensive cooler.
I used a very in-expensive Hyper212+ on mine and ran it for years at 2.9 GHz...without any stability issues prior to moving on.
If AC is your primary goal, I'd up the RAM a bit and turn off all apps in Python.
It'll run much smoother.