In this case your equation is true since the F3-12800CL9S-2GBXL is a single 2GB module and two of them make up product F3-12800CL9D-4GBXL BUT the F3-12800CL9D-4GBXL is a matched kit meaning it is guaranteed to play well with one another whereas the individual modules may cop attitudes towards each other...
In reply to both of your emails, the dual module memory kit does not
automatically equal two single module memory kits. The dual module memory
kit has been tested for compatibility between the modules, but the single
module memory kits have not been tested for compatibility. If you use two
single memory module kits in a single system, you might encounter
Regarding F3-10666CL9S-4GBXL, it is compatible with your motherboard on its
own. If you need 8GB, please purchase a dual module memory kit to avoid any
Feel free to contact us again if you have any further questions. Thank you.
Tech Support Team, F.A.E.
G.SKILL International Enterprise Co., Ltd.
8F No. 69 DongXing Rd., Xinyi District
Taipei City, 11070 Taiwan (R.O.C.)
+886 2 2766-7889 #861 http://www.gskill.us/forum/
Actually I stand by what I wrote as the validity of the equation - same as what they say since the F3-12800CL9D-4GBXL is a kit of two modules - and should be purchased as a kit. The eMail you quote there is a different single 4GB module (F3-10666CL9S-4GBXL), not at all a valid comparison.
I would hope two 1600MHz modules wouldn't be the same as a single 1333MHz module anyway.