Dual-CPUs work EXCELLENTLY on a PC. My father runs three dual-CPU workstations at his office, and I can personally say that the performance gain from the extra CPU is all-too-noticeable. Granted, OS X might have a slight upper-hand in dual-processing configurations, but who's going to win: Dual 1.25GHz G4 or Dual P4 Xeon at 2.6GHz? Which is going to cost less?
(BTW, you wouldn't buy a dual-processor rig for gaming on a PC, maybe two or three games will actually take advantage of a second processor)
Oh, and it is 800MHz system bus? Wow, I thought those were only optimistic rumors. In this case, I cannot see how a 970 configuration could beat a P5 configuration in any test, especially with an Altivec unit that is less potent. Granted, the move to 64-bit processing allows some information to travel faster and for memory allotments to increase, but I do not think we'll see enough of a boost to beat the P5 of 2003.
In either case, I think comparing the G4 to the P4 and the 970 to the P5 is irrelevant. If you can do your work on a G4 450, consider the 970 a God-sent item. If you've been working with a P2 450, the P5 at 4GHz is a fantasy that can be fulfilled. I don't see why we compare Apples and Oranges, whichever system you PREFER, you should buy. Only worry about speed if it's actually an issue. For example, in my house are two Win2k machines at 866MHz and a single iMac at 400MHz. I use the Win2K machines for my web design and programming, while my family uses the iMac for its simplicity and software selection (e.g. iMovie, iTunes, iPhoto). Speed is hardly a part of the equation for my family, so they've clung to the iMac; I need speed, so I've hung on to my relatively quick P3 system.
EDIT: Some quick links
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http://www.geek.com/procspec/intel/prescott.htm
Possible P5 with SSE3..... (drools)
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http://www.geek.com/procspec/ibm/power4desktop.htm
The first chip in a long time to make a PowerMac worthwhile.... (drools)