I switched a year or so ago and had similar confusion about relative performance of different Macs. Things to note about performance in the Mac world:
There are so many factors, people often skip trying to calculate it - it's easier to just test the machine - since there are so few varieties of Macs compared to PCs, you can actually get real answers about what your performance will be for specific apps on specific machines.
MHz speeds up everything; but it's not the only thing.
G4 vs G3. G4's Altivec instruction set gives a great boost to audio, video, graphical - anything that deals with big numbers. G5 even more so. The rule of thumb used to be that G4 is for pros, but these days "digital hub" apps love the G4 and so does the Mac OS X user interface.
L2 cache - this is precious - speeds up most things significantly but for some things makes little difference (think PII vs. Celeron).
L3 cache - a little less precious - you need lots more of it to get the same speed boost. (but hey, if you can have lots of both ...)
RAM speed (133 MHz to 800 MHz) - the larger the files you deal with, the more you'll notice this.
Hard drive speed - look at spindle rate (rpm) and cache size and this affects load time for apps/files
So - 500 MHz G4 with 1MB L2 cache, vs. 800 MHz G3 and a faster RAM speed ... these kinds of things will drive some people nuts because there's no formula except perhaps for one specific use. Better to just tell MacDoc or another guru what you want to do with your computer and let them choose it for you
Or go demo some machines at Carbon or a Mac User Group.
At least you can usually sell Macs used and get a good resale value (think Honda Accord), so it's not that risky of a situation.