Take a look at Cinebench benchmarks. The one's I've seen show *much* better scalability on 2-CPU Macs compared to 2-CPU XP/2000 systems. Just as Billy Boy didn't think we needed more than 640KB of memory or a LAN (DOS days), he also didn't think we needed more than a single CPU (these days), so XP/2000 is misearable at threading and SMP.
As a data point, I can run ffmpeg on a dual G5 and get 1.5 CPUs, while the rest of the system (UI, networking, browser, burning a DVD, listening to iTunes, looking at photos, ...) respond crisply. On my wife's XP box I can't get any real multi-tasking; I burn a DVD using Nero and I'm hard pressed to get another App launched and working in a way I can use it. In fact, on her macine I'm *afraid* to try to do anything else for fear of blowing the burn. I've had that happen on XP, but never on OS X. In short, Unix/OS X was made for multi-tasking and, since the early 90s (not OS X), made for SMP.
I can tell you that I Fold twice as much with my G5 2.5DP than I would with a single CPU Mac.
Last, but not least, look at the cost delta. The new 1.8 GHz SP G5 PM is only 25% less (list) than the DP 1.8. Even if you were only getting 50% better performance from the DP configuration, you have a significantly better price/performance ratio. 'Nuf said.