Some memory manufacturers have a Memory Finder/Compatibility tool, such as
Kingston and
Corsair and
Crucial..Retailers such as
Newegg also sometimes have a Memory Finder/Compatibility tool..Some of the Memory Finder/Compatibility tools you need to select "Show discontinued motherboard models" or something to that effect..You can also perform a forum search for "790i RAM" (or similar phrasing) and see what other people use..Identifying compatible memory is one thing, finding it in stock at a reasonable price is quite another.
It may or may not take just a little BIOS tweaking to get 8GB functional, a lot of people ran 8GB on that board..The RAM should manually be set to the advertised speed, timing and voltage specs in the BIOS, not the default Auto settings..The default Auto settings will run the RAM at the JEDEC specs programmed into the SPD chip in the RAM which is usually slower and with lower voltage and many times causes issues.
There's some people (few and far between) that have run up to 16GB in a 790i, but a little tweaking is normally needed.
The 790i (which I used to have too, FTW though) is quite picky about RAM, so no guarantees..The higher voltage stuff (1.8 - 2.0 volts) works best..The lower voltage stuff (1.5 volts) can, but not so much..Running RAM faster than the FSB will do you no good and may cause issues..If you're not gonna overclock the stock 1333MHz FSB, just get 1333MHz RAM unless you wanna run 1600+ MHz RAM at 1333MHz.
That's about all I can tell you at this time..Good luck.