In all honesty lets have a look:
2x256 = Potentially benifits from dual channel, potentially suffers a CR2 pentaly
1x256 = guaranteed CR1, potentially does not benifit from dual channel
For most people, they wont notice betwqeen dual channel and single channel much so you might as well go 1x512 because the CR1 advantage (and minimal potential losses) as well as keeping an upgrade path open