Is a single Dual Core really like a dual processor?
I've got a dual proc machine here I'm playing with and I've got a mamouth SQL query that's got one CPU locked up at 100%, yet the machine is still completely responsive and I can do everything I need to do on the other proc.
I've never looked closely at how the hardware is configured for each, but loosely, I would say behaviour should be similar. In both cases, they have to share resources (memory and disk). The major difference might be the arrangement of internal caches but if the CPU cache is duplicated, I would expect performance to be very similar.
Having said that, a single processor system might do better than you think - it depends on how the task scheduling works, and, to some extent, how the software is written (and the nature of the task). For instance, a big job could be scheduled to run with idle-priority. In this case, the system should still be responsive whilst CPU usage for the major task remained at about 95% or more.