Please take a look:

http://www.slizone.com/object/slizone_build_mem.html

Interesting to see, that according to this table, 790i ULTRA doesn´t support 2GB modules running at 1800Mhz and at 1800Mhz can only use 2 slots.
So in teory, if we want to run our memory at 1800Mhz,or more,we are LIMITED to 2GB!!!

i´m wondering if someone around here is running 4GB (2x2GB or 4x1GB) at 1800Mhz?