Speed or Quantity?

I am running Vista home prem. I currently have 2 gigs of 667 ram ddr2
I have 1 gig of 566 in two 512 sticks.
What is better to continue to run 2 gigs at the faster speed or install the other and have 3 gigs. but lose speed?
I have 1 gig of 566 in two 512 sticks.
What is better to continue to run 2 gigs at the faster speed or install the other and have 3 gigs. but lose speed?