You only need matched pairs when doubling your FSB speed (2x400Mhz FSB)
Huh? This makes pretty much no sense at all. FSB speed and matched DIMMs bear no relation to each other whatsoever. I don't even know what 'doubling your FSB speed' means, unless you're misunderstanding what DDR is.
DDR RAM does not need to be in matched pairs. Most modern chipsets (and of course AMD CPU's) can enable faster memory speeds by reading and writing 128 bits at a time if you have a matched pair of DIMMs (a single DIMM module transfers 64 bits per access).