simple memory conf q

[scorman]

This is probably a generic question:

I have read that on an AMD 2800+ system, that having (2) 256 MB DDRs
are better than one 512MB, since the DDR dual function requires two
diff banks.

I have an ECS N2U400-A board which has 3 DDR slots and presently I have
a single 256MB ...should I get a second 256 to make the pair, or should
I get a 512 to just have more bulk memory?

Last question, if I have (2) 256, and I add a 512 later for the third
slot, do the pair still function in the dual DDR configuration?

TIA,
Stew Corman