It does not matter, you can overclock the video card and the processor.
Think of it this way, the video card is basically a processor for videos. You can overclock the processor of the video card.
A processor...well...it processes stuff. You can also overclock that. They are completely separate of each other.
The inability to overclock the i3 has nothing to do with the video card.
Here is an article that will outline the kind of performance you are probably going to have:http://www.tomshardware.com/reviews/build-gaming-pc-overclock,3159.html