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Without 'tweaking the BIOS' you'll never get optimum performance. 'Best' to me would be at least 2X4GB, and without any modifications RAM runs at 1066 MHz. The simplest is to get XMP RAM per the QVL list that's 1333 MHz to avoid the DRAM O.C. Profile. Example F3-10666CL8D-4GBHK which is a 2X2GB XMP kit.
Here's a list of Tested RAM from G.SKILL ->
http://www.gskill.com/configurator2.php?pid=2&model=170
Also, you can refer to Certified {QVL} list ->
http://usa.asus.com/product.aspx?P_ID=j02KziJq95KbCQNm
'Most' folks here can easily help you set your RAM from the BIOS.