Has anyone ever seen this bizar memory problem

ok my machine is a core i7 920 gigabyte mb. i am running a 64bit OS (windows 7 x64) however the computer does see all 6gb of memory but says only 4gb is usuable. whats even more bizzare is that windows see's there is 6gb ram but bios or that black scvreen when you first boot only see's 4gb? how is this possable that windows see's more then bios and in fact there is 6gb ddr3 1600 memory installed.

please help figure this problem out or diagnose it

the memory is gskill ddr3 1600 cas 8 however it is running at 1420mhz with cas 7

only thing i can think of is i did have a 2gb ram disk using ramdisk software but i formated everything back to zeros when i re-installed everything

System congfig utility
boot tab
advanced options
uncheck max memory if its checked
Also your ram settings could be unstable
try mobo default ram settings and see if your ram is reported properly
http://www.cpuid.com/softwares/cpu-z.html
and
http://www.memtest.org/
are very helpful diagnosing ram issues