Below are benchmarks for Far Cry 3 and BioShock Infinite using various GPUs that are relatively inexpensive to moderately expensive. The CPU used for the test is an Intel Core i7-3770K @ 4.6 GHz. I am not going to check the price for each card, that's something for you to do if you are trying to find the best bang for the buck. I am pretty positive that the least expensive GPU in the chart is the Radeon HD 7790 and the GTX 660 Ti is most likely the highest price GPU in the chart.
The resolutions are actually higher than 1600x900 and 1920x1080 so you can expect a little higher performance if those lower resolutions were used. I would say probably between a 5% - 10% at most improvement in performance. As you can see in the below charts Far Cry 3 is pretty demanding, but it certainly is not the most demanding game with graphic quality set to max
Below is Bioshock Infinite. Even with maxed out graphics the game is pretty easy to run. The frame rates are more than double compared to Far Cry 3.
Whats your budget you willing to send on a gpu but at 1080p i would recommend a gtx 760
I need to know the difference in requirements/price so I can decide whether or not it's worth it to me to pay the extra money.
In other words, what I'm willing to spend depends on the answer to my question.
For the price and best bang for the buck you could go single or sli 760s which is prob the route i would go for 1080p. A gtx 770 for a single gpu would be more powerful the a 760 if you didnt wanna go the sli route