The PowerPC 7410 can apparently be had in 600-700MHz varieties, and it's these that you'll need to use if you want a B&W G3 or Yikes! G4 on a full 100MHz memory bus.
For some reason the later models with PowerPC 745x variants use FPGAs to interface to the memory bus, and these FPGAs can't operate faster than 66MHz, thus the requirement to drop your bus speed. Oddly, the same 800-1000MHz 745x chips don't appear to require FPGAs in the older beige Macs.
The difference between the 7410 and the 745x chips is the use of full-speed L2 cache and the inclusion of L3 caches on the 745x where the 7410 has only half-speed (at most) L2 cache. You'll still get a good performance boost out of the 745x-based upgrades even with the memory bus speed drop.
The best 100MHz bus-based chips for these are the 750GX, which has 1MB of full-speed L2 cache in addition to core speeds up to 1.2GHz. These are oddly hard to find though, even though they should have been less expensive than equivalent G4 upgrades. Possibly because, without AltiVec, they struggle with some of the pretty stuff under 10.4 Tiger and can't be used at all with 10.5 Leopard.
If I remember correctly, they're more "traditional" network devices, in that they're mostly concerned with doing packet-level stuff and doing it well. I'd love to get a FastPath, maybe one day I'll find one . It'd be interesting to see whether there's a performance differential brought on by the GatorBox's "do everything" approach.
Yeah it's kind of a tricky subject, at least as far as use in Macs goes: sometimes they may be pin and signal compatible but ROMs don't recognize the new chips so they won't work. Upgrade suppliers such as Sonnet, Newer, and Daystar had custom firmware updates for some models that used new chips, such as the 7448 upgrades for late-model PowerBook G4s and G4 upgrades for B&W G3s (though these models supposedly worked with G4s until Apple blocked them with an "update", thus requiring a 3rd party firmware patch to reenable the use of a G4).
Chips that are supposedly pin-compatible:
QFP 240: PowerPC 603, 603e
CBGA 255: PowerPC 603e(v), 604(e)(v), 740
CBGA 360: PowerPC 750(L), 7400, 7410
CBGA 292: PowerPC 750FX, GX
CBGA 483: PowerPC 7450, 7455, 7457
CBGA 360: PowerPC 7440, 7445, 7447(a)(b), 7448 [though it's still a CBGA 360, it's not compatible with the earlier CBGA 360 used by the 750/7400]
Maybe one or two of the PowerPC 970 variants are pin-compatible but they're epoxy-sealed to their carrier boards so they're very difficult to rework. Not that there would be much reason to do it anyway.
The PowerPC 750CX has a unique BGA.
Though they use the same 304-pin QFP package, the PowerPC 601 is not pin-compatible with the 601v, nor with the QFP variant of the PowerPC 604.