wikipedia on dual channel architecture wrote:in order to achieve this, two or more DDR/DDR2 SDRAM memory modules must be installed into matching banks, which are usually color coded on the motherboard. These separate channels allow each memory module access to the memory controller, increasing throughput bandwidth. It is not required that identical modules be used, but this is often recommended for best compatibility for dual channel operation. It is possible to use a single-sided module of 512 MiB and a double-sided module of 512 MiB in dual-channel configuration, but how fast and stable it is depends on the memory controller.

If the motherboard has two pairs of differently coloured DIMM sockets (the colours indicate which bank they belong to, bank 0 or bank 1), then one can place a matched pair of memory modules in bank 0, but a different-capacity pair of modules in bank 1, as long as they are of the same speed. Using this scheme, a pair of 1 GiB memory modules in bank 0 and a pair of matched 512 MiB modules in bank 1 would be acceptable for dual-channel operation.