Studio XPS 435T RAM upgrade

I'm trying to upgrade my RAM in my Studio XPS 435T from 12G to 24G.  I've installed 6 sticks of Crucial (Ballistix Technical) 4G RAM.  I check my system and it says Installed 24.0 GB (16.0 GB usable).  I went into the CMOS Setup Utility - it says Memory Installed - 24576MB and Memory Available 24568 MB.  Memory speed is 1066MHz.   I don't know why there is a difference of 8 MB.  Is there a different brand that would be better?  If I have 24GB of RAM installed, why is the system saying that only 16 GB is usable?  Any help would be greatly appreciated.