:: Which memory works best with GA-MA790GP-UD4H ?::

This is what neweggs specs say

http://www.newegg.com/Product/Prod [...] 6813128384

Or you could actually go to gigabytes web site and get the officially supported RAM list.
http://www.gigabyte-usa.com/FileLi [...] p-ud4h.pdf

Lots of memory that will work never makes the approved list - it's endemic to the industry. For GBs, what happens is the approved memory list is made up when the MOBO is introduced, from sticks they have been provided samples of, and never updated thereafter; in addition, many of the memory manufacturers that you'll see there you've never heard of, as (and I've said this before) I'm sure you can pick them up at any gas station in Taiwan, but they're not to be had here. If you can find some that's 'on the list', and you can get it, at least you know they got it working, somehow. If I'm not mistaken, AMDs do not read extended memory profiles to set RAM faster than standard JEDEC spec (which, for DDR2, is 800) - you have to set the faster speed either in your BIOS, or by using AMD's OverDrive software, and, again, if I'm not mistaken (I'm not an AMD expert, or friend...), you are limited to two DIMMs if you elect to go with 1066 or faster...

I really wouldn't consider this something "wrong with it...". It's just an underlying 'feature' of the AMD on-die memory controller architecture. All motherboards, regardless of chip manufacturer, have difficulties with more than two DIMMs; for the Intel 775 sockets, you pretty much always need to go into the BIOS and 'bump up' the northbridge voltage to accommodate four. I'm sure this will eventually be addressed, but it is not yet seen as a real impediment, as very few people actually have a need for more than four Gb of RAM; and the ones who do, are often (but certainly not always) technically savvy enough to make the adjustments. The reason I posit that it will get fixed is Wirth's law: "Software is getting slower more rapidly than hardware becomes faster." Program 'feature bloat', the wider availability (and desirability) of sixty-four bit operating systems, as well as the growing popularity of video editing and transcoding, will soon make eight gig the standard, rather than the exception...