1333Mhz ram will work for sure. I know people that use 2x2GB OCZ Black Edition 1600MHz 8-8-8-24. the best way is to check on asus website. your ram is in the supported list, so u wont even need to underclock it, u can run it ~1600Mhz, use the ram ratios.http://www.asus.com/product.aspx?p_id=b6r5qxwnoajzwcux
- go under Momeory/device support and download the zip, it contains .pdf files with asus memory support list for your mobo. u will see your ram there.