My 1520 has .9gig of memory.  i assume it's two 512s.  I have one Dell 2gig upgrade module.  My manual notes "in order to take advantage of the dual channel bandwidth capability both slots must be populated and must match in size. Can I choose to either:

1. populate 1 slot with 2 gig and the other with 512meg or
2. populate slot A with 2 gig and leave B empty?

What will be the effect of not being able to take advantage of the dual channel bandwidth?

Thanks