This board has 4 slots, but you can't utilise them!
In the manual, it tells you to use matched modules and how to arrange them for dual channel,
then there is a great amount of disclaimers. It has a table of dodgy configurations to avoid with
FSB 1333 MHz, and one of those is 4 single-sided modules. There is also a paragraph stating
not to use 2 double-sided DIMMs in same channel, so you can't have 4 DS modules either!
I did some experiments using DIMMs that were all same speed (677 5-5-5) and found yes
it will not detect all modules for most configurations. These modules are all good - they
were detected if plugged in alone.
So it seems to get 4 GiB you have to use 2 x 2 GiB modules.
But even more distubing, when I tried 2 x 1 GiB DS and 2 x 512 MiB (SS), it did not even boot
AND THE CPU FAN STOPPED AFTER 4 SECONDS!
That is serious flaw.
I know it used the crappy crippled P31 chipset, but Asrock made a board using the same
P31 that would run 8 GiB of RAM.