The a7V8x is based on VIA KT400, that is single channel DDR with a (theoretical) frequency of 2x200MHz, which equals the 400 in DDR400. To get the fastest dual channel DDR you need the nVidia nForce2 (which is nothing to do with graphics) chipset on a motherboard, for instance a7N8x - only one letter difference but it's important! As far as the a7v8x goes, you'll find it runs faster with good quality DDR333 memory (for instance Corsair) instead of DDR400. Just trying to give you the benefit of the money I spent to learn that! There are good benchmarks on this site which let you compare performance of the different types of memory.
The dual channel memory itself is just normal DDR though, as the chap above says, you must buy a matching set (same type, same manufacturer) to get the benefit. The difference is the chipset on the mobo.