At the moment Im running my C2D with 2 512MB DDR2-800 C5 Stick from corshair, I have now come across 4 512MB DDR2-667 C4 sticks from corshair that I may use, Should keep the DDR2-800 in there and just two of these sticks in or should I replace the DDR2-800 with the DDR2-667 sticks?