You'll need to replace all of the RAM if you want to run them at a higher speed. But that is if your motherboard supports that speed or if it allows you to overclock to that speed. If the new RAM is 1,666 MHz, they will all run at 1,333 MHz because the BIOS will downclock them to match your old RAM by default. Plus, according to your motherboard's specs (all revisions) on Gigabyte's website, the highest you can clock your RAM to is 1,333 MHz.
http://www.gigabyte.com/products/product-page.aspx?pid=3610#sp
That doesn't mean that you can't use 1,600 MHz RAM. If the higher clocked RAM is cheaper than the 1,333 MHz RAM (unlikely), you can still use it but it will run at 1,333 MHz. I have seen that 1,066 MHz RAM is sometimes more expensive for some reason than 1,333 MHz. Depends on what brand it is and the timings. Could also be due to supply and demand as most newer systems use faster RAM and the 1,066 and below is being phased out.