General rules:
Determine DDR___ rating by multiplying actual speed (in MHz) by 2 (effective clock rate, since information is sent twice per clock cycle).
Multiply DDR___rating by 8 to get PC____ rating (in GB/s, since it measures bandwidth).
Therefore, things like PC3200, DDR400, and 200MHz (used in the context of DDR memory) are all one and the same.
Whether or not you can use dual channel is up to your memory controller (chipset usually, but CPU on a K8-based system) and board if for some reason either one rejects some sticks for no reason. You need to have pairs of memory to use dual channel (any number of pairs up to the maximum number of sticks the controller supports is fine, though), though on NF2-based systems (where dual channel makes no difference unless you use the NF2 IGP integrated GF4MX graphics) I think you can do some weird 3-stick dual channel with, say, 2x256MB and 1x512MB.