I have always heard (but have no direct experience) that GB boards, in general, 'love' mushkin memory, but I think that's mainly applicable to the Intel socket 775 pieces; from AMD I don't know! Place to ask is probably over at TweakTown:
http://forums.tweaktown.com/f69 Post to 'LSDmeASAP', if he doesn't know (he's pretty much an 'Intel guy', too...), he knows someone who does!
I upgraded my MB and CPU to GA-MA785GM-US2H and Phenom II X2 550 Dual Core Black Edition and am using 2 x 2 GB Mushkin dual channel memory. The computer works fine (so far).
By the way, I don't see LED on the MB and no beep when the computer boots. I'm still searching to figure it out.
Couple things to know -
note on page sixteen of your manual: "If two memory modules are to be installed, it is recommended that you install them in the DDR3_1 and DDR3_2 sockets."
First thing to do is the BIOS' "Load Optimized Defaults"; that done, check your memory speed - I'm not sure, but I don't think AMDs support EPP/XMP memory profiles, so you may need to set the memory multiplier manually (to "X6.66" for 1333)...