* Intresting thoughts. But, why would he have to replace the 256MB modules? If you have two 256MB memory modules in an nForce2 board and plan to upgrade to 1GB of memory, just buy a 512MB memory module and configure them as such: the 512MB memory module in the memory slot that appears to stand alone and the two 256MB memory modules in the two remaining slots.
* In the case where only one channel is filled, one memory controller does all the work while the other memory controller remains idle. It would be best to place a balance workload among the two memory controllers. In which case the two memory controllers could work together in order to increase bandwidth and decrease latency. But, you are right: getting a 512MB module now and taking that small performance hit and adding another 512MB module in the near future is another option.
<b><font color=blue>Logic over all.</font color=blue></b>