As the other post implies, dual-processors would make a HUGE difference when running dual-processor-aware applications, such as PhotoShop. Also, OS X takes advantage of the dual-processors, making the whole computing experience a LOT faster under OS X.
However, under OS 9 running applications that AREN'T dual-processor-aware, the 867 is, by my calculations, only 8% faster than the dual-800 MHz version, which would only be running one 800 MHz processor. That's a negligible amount in my opinion.
With the future of Apple's software and hardware, I think it would be a hands-down good move to get the dual 800 machine. I say this because I think that machine would last you years and years, and based upon that conclusion, I think at one point you'd eventually move to OS X... and the dual-processor version would make a world of difference compared to the 867 MHz version.
Here's my plan once I win the lottery: buy a stripped-down dual-800 machine and upgrade it with aftermarket parts. Get the smalled hard drive there is (40GB I think) and add a 60 or 80GB drive as a slave. Get the GeForce2, just because it'll run what I need it to run, and I can always get a Radeon or GeForce3 later on... knock the RAM down to the minimum, which I think is 256MB (maybe 128 now... I remember earlier 256 was the lowest they'd letcha go) and then add 2x512MB chips. That would get me through the year 2005, I'm betting...
I've had my G4/400 PCI machine with 640MB of RAM since the G4s were introduced. I originally priced a G3/450 machine and ordered it and shortly got a letter back from Apple saying that the G4s were out and they were no longer selling custom-configured G3s... so I got a G4 WITH a monitor for the same price... this machine, albeit slow in comparison to the new machines, works absolutely great. I use PhotoShop heavily, and boot back into 9 when I need to -- of course, yes, I'd love a little speed boost, but it's fast, stable, and fun to work on.
My point: if you can afford the dual-800 machine, I say go for it. That investment, I'm betting, would last you 5 years minimum... good luck with your decision! Besides, if you got the 867 model, you'd always be wondering, "What if I had gotten the dual-800?" I did the same thing with my VW -- the guy TOLD me not to drive the V6 othewise I'd always be yearning for more if I bought the 4 cylinder... and I am yearning for the V6 while I putt around in my 4-cylinder...
Nevermind... don't make a decision. There's only one decision -- if you can afford it, get the dual-800.