To operate in dual channel mode, you need two matching modules - installing one 1G and the other 2G will break the dual channel configuration and cost you 10-15% in overall performance.

Thanks, husky0894.

Ok, so I think I get it now.  If my machine has four slots (2 pairs of slots, 1 and 2, and 3 and 4), I want to fully occupy either pair, or both pairs.  And, each pair (Channel) that's occupied should have matching memory (a 'kit') in both of the paired slots.  They should match in both size and speed.  But, I'm not clear on density.  I did read that desktops/laptops should use low density, as the high density memory is for servers.   Out of curiosity, will this set up take advantage of dual channel mode.  Can density be deciphered from these descriptions?

Channel A, Slot 1: 1GB, 667M, 128x64, 8, 240, 2Rx8 (factory installed)

Channel A, Slot 2: 1GB, 667M, 128x64, 8, 240, 2Rx8 (factory installed)

Channel B, Slot 3: 512MB, 64x64, 667, 64x64 (Crucial recommendation)

Channel B, Slot 4: 512MB, 64x64, 667, 64x64 (Crucial recommendation)

To operate in dual channel mode, you need two matching modules - installing one 1G and the other 2G will break the dual channel configuration and cost you 10-15% in overall performance.

So if i have 1.5GB ram right now on a Latitude D810 Notebook does that mean I am loosing that 15%...  Just upgraded today..  Used to be 1.0GB..  So I bought a 1.0GB memory stick thinking I would just get the max of 2.0GB..  Not knowing that they used 2, 512MB sticks...  Giving me 1.0GB..

I already bought the other 1GB..  Waiting for delivery...