Optiplex 755 MiniTower RAM Question

I have just installed a new motherboard, DELL Optiplex 755 Motherboard SMT LGA755 Small Mini Tower OEM 0GM819 GM819.  All is working fine except my memory.  I have Windows 10 64 bit OS.  I have ULTRA ULT31908 2 x 1024MB PC-6400 800 Mhz DDR2 installed in all 4 DIMMS.  This memory was pulled from a working machine. The Bios only recognizes 1GB in each DIMM at 667Mz.  I currently have an Intel Core 2 6700 2.66Ghz CPU that's clocked at 1066Mhz..  I know the board can have 8Gb of memory installed.  Why is the BIOS only recognizing 4GB at 1GB per DIMM and reduced to 667Mhz?

I am thinking to upgrade the processor to an Intel Core 2 Quad Q9550 2.83GHz clocked at 1333Mhz.  Can I use an upgraded memory and make the above a mute point?  What would the fastest compatible memory be.