Your ram, if its good, will support 2 easily...
Reduce to 2.5 or 3 if the ram is really naff...
(i guess a speed increase of 6 - 10% might be possible)
But your motherboard/Bios might not support Cas rating (it will be preset on `Auto`)
Have a read of `BIOS for Beginners`
here.(no mention of CAS there though)