Notice I didn't say performance gap, and I did that advisedly. Pundits on both sides incessantly scream about comparative speeds between the processor lines, bashing each other with various benchmarks. I used to use my thoroughly unscientific "half generation rule," which states, more or less, that a given PowerPC would seem to be about halfway between the comparable Pentium chip and its next generation, with about a 25% performance boost in real world tasks at the same clock speed. (You'll always find some tasks in which a Pentium is more efficient, and Apple fans love to point out pathological cases where the PowerPC can run a specific Photoshop filter 500% faster, but c'mon, who cares.)
A friend of mine used to talk about audio amplifiers and reminisce about his great old NAD amp, which had 25 watts a channel. "But they were NAD watts," he defended, and up to a point he was right. It was great amplifier design. But that only gets you so far, and of course that's the PowerPC's problem. It turns out that the venerable 686 core used in the Pentium Pro, II and III, as well as the 786 followup (Pentium 4), can be pushed much, much faster than a G4 can. If we follow that unscientific 25% rule, the fastest Mac is the rough equivalent of a 1.8 GHz Pentium III--nothing to sneeze at, but nothing too amazing, and the fact that you can only get it in a $2700 dual-processor configuration will cool most people's heels quick. That the G4 isn't efficient in multiprocessor configurations doesn't help things, either. And the G4 is the rough equivalent of the Pentium III, anyway--not the P4.
The rumored specs of the PowerPC 970-based G5 bring the buzzwords the Mac world has been lacking into the fold, and more--AMD's "HyperTransport" bus architecture, a front-side bus even skeptics will admit sizzles, serial ATA, high-speed RAM, so on and so forth. There are even more conflicting reports on the clock speed than normal for rumors; if we assume a 1.8 GHz speed, that's the rough equivalent of a 2.3 GHz P4.
Again, nothing to sneeze at, but critics are already harping on how it's possible to get faster cheaper PCs now. Apple defenders will probably passionately point out technical details--as a 64-bit workstation CPU, the PowerPC 970 is more comparable to AMD's long-delayed Athlon64 than the Pentium 4, and the 970 supports multiple processor cores on a single die, so that a "dual core" 1.8 GHz 970 might well perform more like a 3 GHz 970 in real world situations. (In benchmarks, it's likely to perform no better than a single-core 1.8 GHz chip, because benchmarks rarely test multithreading.)
But I think it'll be enough for the people who are actually going to be lining up to buy G5s anyway. Whether it pulls in people sitting on the fence will be a question of price. My bet is that they're going to completely replace the PowerMac line at comparable price levels, and it'll be possible to get something that's at least 1.8 GHz at the $1999 price point.
If I ran the zoo, I'd also take the opportunity to introduce an "iBox": a G4-based machine akin to the barebones Shuttle PCs, sold with just case, power supply, motherboard and, say, 1.25 GHz CPU for $399, and in a just-add-monitor configuration with hard drive and RAM for $599. And only sell the "bare" version at dealers like Fry's--Apple stores would have only the complete iBox. Like the Shuttle PCs, the machines would have sound and video on the motherboard, but you'd be able to plunk in replacement PCI cards as upgrades if you wanted. Third-party dealers feel like they're being at least slightly catered to again, kit-bashers would finally feel like they're being catered to, people screaming for Apple to get into the low end finally would be shut up, and it'd be just enough of a premium over PCs that people who aren't happy unless they're bashing Apple for being low-speed and high-cost would be reassured the world hadn't changed.