What is your budget? There are many great choices depending on how many PCIe/RAM slots you need.
In my current system I am using the x4 965 BE overclocked to 3.8GHz (stock cooler) on the Asus M4A88T-M/USB3 with 8GB of Kingston DDR3-1333 (4x2GB) with a Zotac nVidia GTX465... it is best bang for the buck if you want something solid and don't need SLI/Crossfire and want the mATX form factor. I also use it with a very small mATX Cooler Master Elite 341 case. http://www.newegg.com/Product/Product.aspx?Item=N82E16813131660