the dual PMs are considerably faster at almost everything. To make them so with EVERYTHING, you should upgrade the graphics card and the RAM right away.
Today's are better than they were, but traditionally, for some reason, the powermac graphics cards have been anemic, even on the 3,000 dollar version. I guess they want to force you to upgrade. This time last year, a 3,000 dollar computer with no monitor and an nVidia 5200 with 64mb of RAM could only mean Apple, because no other company in the world would expect someone to buy such a thing...
well, yes, the new dual-core PMs are very nice. Heck, the quad-CPU ones are ultra-nice. With a 7800 and 3-5GB of RAM, maybe a nice RAID array, and you're looking at a NICE chunk of machine that will do anything you want it to do (short of games) faster than any normal PC would ever think about doing it. Unless you've got an opteron with a 4 processor config. Or maybe one of the new dual-core FX chips...they blaze...