My budget is around $750. I want a gaming pc that can play most bigger game on high or ultra like GTA V or BF4. Thanks.
Go to pcpartpicker.com and play around with configurations. Get an Intel i5 or one of those AMDs if you want to same money for a better graphics card. If you live near a Microcenter, they have great mobo/cpu combo deals. You only need 8 gb RAM. I suggest a 60 (or 120) gb SSD with a 1 TB HDD for fast boots/loading times. And for cases, get one in the 50-100 range. You can get good ones in that price range.
GFX, I have no clue what the latest and greatest ones are. You want a good one for your gaming needs so this will be most of your budget. Go read the MaxPC blog, Tom's Hardware, or Anand reviews.
You gave only a budget so that's as far as I can help. Do you need keyboard, mouse, monitors? Good luck and show your build here. We can tell you if it will work out.