If you're running OS 8 or 9, only specially enabled programs can really take advantage of the dual processors... Photoshop is one of them, much to my delight. OS X, on the other hand, is supposed to have native support for multi-processor systems. I'm waiting to try out OS X until more software has been released natively for it.
It didn't take long for all my favorite publishing/graphics software to come out PowerPC native (way back when! ;) ), so I'd best start saving for a new home computer soon...
I've not noticed any sort of slowdown with non-enhanced software running on my 450Mhz dual processor G4... and the speed increases with 'enabled' software like Photoshop are very noticeable.
The dual Pentiums are a different chip architecture, running a different operating system, etc., etc... A diesel engine vs. a gasoline engine vehicle would yield much different performance characteristics, even if both of them offered the same "horsepower."
All the benchmarks I've seen on the DP Macs (running OS 9) show a marginal speed improvement with non-enabled software, and a very significant improvement with software support. I can only assume having native operating system support for the DP arctiecture would yield much more significant improvements.