RAM Config: (2) 512 MB + (1) 2 GB

Your proposed configuration should work, but only in single channel mode.  In order for the memory controller to work in dual channel interleaved mode you must install matching pairs of memory modules into the slots having matching colors on the retention latches.

The loss in efficiency by not having a matching memory set in channel A is not great, and is usually outweighed by the fact you have more memory space for use by the processor.  The increase in speed made possible by having a matching set of memory in channel A is less than 30%.  Vista, for example, will perform much better with the 1 GB stick in place compared to being without it, and if you had instead put another matching pair of 512 MB sticks in channel A you would hardly notice the difference between that and your existing 1 GB stick.