upgrading RAM, quick question

[S.A.S]

hi all, was thinking to upgrade to 6 gigs of RAM finally, and plunge into the purchase of win7 64bit, my question is, to fill the other 3 RAM slots, does the memory have to be exactly the same as my original 3gigs? or can i get a totally different version, obviously sticking to the same make of corsair?

 

[S.A.S]

would it be better just to sell the 3gb i have and just purchase a 6gb kit? i think would be less expensive tht way and i then have the future upgrade of another 6gb,