You probably are correct: if you are tunning older, slower RAM with faster modules then the mobo will probably default to the loser speed. Take out the old RAM and try the new to see if it runs on your mobo.
First, it's never a good idea to mix-match RAM, next 2x4GB + 1x4GB kills Dual Channel. Use the first (3) DIMM slots. Together, they cannot run DDR3-1600 and you're stuck to DDR3-1333 CAS 9-9-9-24-2T @ 1.50v
Q - What MOBO?
Memory Remapping -> Enabled (required for >4GB)
Keep the DDR3-1600 in SAME colored DIMM slots
Place the DDR3-1333 in OTHER colored DIMM slot but closet to the CPU
Typical problems include setting Maximum Memory; set MSCONFIG as shown:
From what I see you are running the 32 bit version of Win7. 32 Bit OS can only use up to 4gig of ram. Note also that the video ram is counted into this total. So if you are using the 32 bit version the system ram available is 4 gig minus whatever video ram you card has.