I have been doing some research on the capabilities of the G4 vs. Intel Chips (checking GeekBench scores, etc.) It seems that MHz for MHz the G4 is just about equal to the Pentium M/ Core Solo Mflops (about 60% of a Core Duo) which is pretty good for a chip whos design is much older.
I am running OS X 10.3.9 on my 400MHz G3 Pismo and the OS itself really zips compared to Window XP on anything less than a 3GHz Pentium 4 or 1.8GHz Pentium M/ Core Solo. Heck even then the slowness of Windows XP on the 3GHz P4 boggles the mind; just click on the volume icon in the System Tray and see how long it takes to bring up the volume slider! And it is a real pain watching Windows try and find the icons on an explorer window.
A 400MHz G3 (with 384MB of RAM at least) is not up to the modern web though; videos are unviewable at 1-2 FPS, active content webpages (such as the new yahoo mail interface) are sluggish, etc. I always keep the Activity Monitor up so when things stall I can see if something is 'working' as the CPU stutters along at 100% usage, trying to delegate what it can to various tasks, or if it has actually crashed.
I really think it takes a minimum 500MHz or so G4 to view the web these days (about the equivalent mflops of a 1-1.5GHz Pentium 4?). With Windows XP you really need a 3GHz Pentium 4 or 1.8 GHz Pentium M or Core Solo to use the web stably. The wife's PC is a 1.7 GHz Pentium 4 (with 768MB RAM and a 'pretty good' 128MB AGP Video Card) and it's OK if not perfectly smooth for web videos etc. My Father-in-law's 3 GHz (1GB RAM, 128MB AGP video card) works nicely on the web. Of course video card, RAM and other things make a difference, but I am looking for 'real-world experience', not technical exactness.
I am looking for ratings of watts and 'real-world' Mflops for later 1.4 to 2.0 G4s for comparison, the only Freescale docs I could find go up to 1 or 1.2 GHz. Processor MIPS would be nice also but seem to be hard to find anymore, maybe they just don't have much 'real-world' relevance.