There is no such thing (in current implementation) as "dual channel RAM".
What you are buying, if you get a "dual channel" kit, is a "matched pair", that need to be installed into a dual channel motherboard acccording to dual channel configuration rules - in the appropriate slot pair, which would typically be one slot of each colour, and with no other RAM.
The "matched pair" do not need to be precisely matched, but using the same brand/type is usually a good idea - but if you pay any more for "dual channel RAM" than for two of the same memory modules as "2 pieces", then you are being ROBBED!
In actual fact, if the "512Mb dual" is a fancy name for a pair of 256's - and it's priced similar to a single 512, it probably is overpriced.
PC3200 should work down to PC2700 or lower speeds