Hi,

If your gift card has sufficient funds for the full price of the game and shipping (if applicable) then your credit card will not be charged. 

GTA4 is quite nice on the computer, but I recall it requiring a lot of resources. Now, my computer is about two years old, but when GTA4 came out, pushing the effects up was quite strainful with a quad core CPU and a higher end video card. I didn't find any noteable issues going through with a keyboard and mouse, but if you prefer the comfort of a gamepad, I say stick with what works!

Enjoy!