Since I have found answers to my problems, I am concluding this thread.
After spending some more days of testing different things and talking to others with similar PC configurations I have found the following:
1) The message "AMD Data Change...Update New Data to DMI!" appears to others as well, and it's normal. I really wish GigaByte would release an update that fixes this, because it doesn't "sound" normal.
2) The SMBus listed as NA in the IRQ POST Bios list is also normal. Everybody with whom I've talked to, had this, and there is a number of videos on youtube that confirms this.
3) Freezes were a memory incombatibility issue or faulty memory issue. As I 've said, I did check the memory modules, together and each seperately, in consecutive memest86+ 4.0 tests, and none found any errors, so it should be something that had to do with the memory clock or sth. The Corsair memory seemed to be more stable @1033Mhz, with some manual settings in BIOS, but would not work well @1333Mhz in dual channel.
Then, I tried the Kingston's recommended in the GA-MA790X-UD4P in 2x2GB dual channel, and everything runs smooth now and @1333Mhz!
So, problem's solved. For now, anyway.