single-ranked vs dual-ranked

[Jason Martens]

Does anybody have a good description for the difference between
single-ranked dimms and dual-ranked dimms, especially what the maximum
memory is with dual-ranked dimms?  I have a system that has 4 2GB
dual-ranked dimms, and I want to upgrade it beyond 8GB.  My
understanding is that is impossible, unless I switch to all
single-ranked dimms, or purchase 4GB dual-ranked dimms (which don't seem
to be available).  Is that correct? 

Also, dmidecode seems to think that I have 6 2GB dimms installed, when I
really only have 4 (/proc/meminfo says 8GB total).

TIA,
Jason Martens