The MDD power supplies are expensive to replace and the single 1ghz was only used in the FW800 model. A dual 867 is definitely faster except if you are booting into 9 where the second CPU isn't used by the OS and your software must be multiprocessor aware to show speed gains from multiple processors. The 1ghz single also runs on the 133mhz bus so you can't swap it into a 167mhz bus model without seriously overclocking it.