Hey all, I just booted up my old computer for the first time in years and I want to breathe some new life back into it.
As i recall, the 790 was quite picky about memory, but it seemed to love the Patriot Viper line. I currently have 2x2gb of 2000mhz Patriot viper memory. My brother happens to be upgrading his (similarly built) computer and is willing to give me his old memory, which is 2x2gb of 1800mhz patriot viper memory. I know that you have to use the same sticks across channels, but can i run the 1800 in the last two channel and just drop the ram speed in the BIOS?

Had some green Vipers in my 790i FTW and they worked flawlessly with my Q6600 at 3.6 GHz and is probably the best RAM for the 790i boards.
 
Mixing them should work fine at the lower frequency and timings of the two..But the RAM voltage is what you really need to be concerned with more than anything..You want the voltage to be the same, ideally, or at least within 0.1V if different.
 
First set the frequency, timing and voltage to the slower/lower RAM in the BIOS, then Save & Exit before installing the additional sticks.

My 790i was able to handle different frequency kits quite well. I mixed and matched several different sticks with hardly any issues. Sometimes it just depends on the combination, although I can't speak specifically on yours.
 
I had a similar setup to bob16314. Q6600 with green Vipers and tried many random sticks with this setup.