After using my DP 1.25 GHz Power Mac for less than a week, I find that I NEVER WANT TO USE A SINGLE PROCESSOR SYSTEM AGAIN. However, when the 970s come out, maybe a single 970 will do a comparable job to the duals of today. I don't know, I'm just speculating.
Anyhow, I love my current machine! Both processors share the load -- viewing the CPU monitor -- no matter what app I'm using. There always seems to be a nice amount of reserve processing power. If I just had a new single 1 GHz Power Mac, the processor would be taxed at all times. VPC, encoding a DVD to Divx, iTunes, Safari - some streaming video, Photoshop and Indesign -- these would tax a 1 GHz for sure, if used all at the same time. My DP does a great job when using all of them simultaneously.
Having a DP system speeds up everything, and I couldn't be more impressed with Apple's implementation of DPs into OS X.
The 1 GHz Power Mac is an embarrassment to Apple's pro lineup -- they should have made the low-end a dual 1 GHz or kept the dual 867 MHz system. They should lower the 1 GHz Power Mac's price to $1999 -- sure it's expandable, but will forever lack in horsepower. It's just not that appealing when looking at how powerful Intel and AMD's chips are. If you want to get ANYWHERE NEAR the oomph of the dark side, at this point in the Mac world, you've gotta go big or go home.
Think longevity; therefore, get a dual Power Mac.