If you are using a single graphics card: 970 Extreme3 R2.0http://www.asrock.com/MB/overview.asp?cat=CPU&Model=970%20Extreme3%20R2.0
If you are using 2 mid-range graphics cards: 970 Extreme4http://www.asrock.com/mb/overview.asp?cat=Specifications&Model=970%20Extreme4
If you are using 2 high-end graphics cards: 990FX Extreme3http://www.asrock.com/mb/overview.asp?cat=Specifications&Model=990FX%20Extreme3
All of them should be within your budget.
The first 2 boards have 2x USB3.0 at the rear and a header for 2x USB3.0 at the front. The last board has only 2x USB3.0 at the rear.