SSD 840 series

I got 2x 4gb installed. Corsair Vengeance 1600mhz When my pc is freshly booted it shows that I have 6gb memory. The memory reduces over time, last night it went down to 800mb. Cache faults are shifting between 0 and 5 million ( very random ) - Is this normal? They are set to 1600mhz in bios and...