Hi newcomer and welcome to the Tom's hardware forum.
What specific rig do you have? What specific mobo?
SOME mobos support DDR2 and DDR3 BUT not at the same time.
I have a hp dv6 notebook, it has an intel core2 duo cpu at 2.20ghz, 4gigs of mem, amd 320gig hdd which runs at 7200rpms. As for the spcific mobo i dont know where would i find that, thnx