hey.. i was wondering whether i could get some help?

i've got a 4 slot mobo that supports dual channel ddr 400.. currently i got 1 512mb double bank, 1 512mb single bank ( both pc3200) and 1 256mb pc2700.

1) is it better if i run da 2 512 on dual channel or add da 256 and run on normal mode?
2)since both da 512 are diff type of banks will da dual channel still work?
3)will running all 3 rams bring down da performance( cos da 256 is lower frequency)?