Whats your prefrence, I have 2 options I just bought a matched pair of 2x1gig sticks. I have installed 2x512 the 2 gigs run at 667 but the 512 run at 533 so...
option 1, Install all four and have 3 gigs 2x1gig in bank 1 and 2x512 in bank 2 so it will run in dual channel or
option 2, Just install the 2x1gigs and leave the 512's out losing 1 gig but running at 667.