The G5 is a faster, more powerful, 64-bit chip. A dual-1.8 is substantially faster than the 500MHz G3. Keep in mind that, as mentioned above, you're not simply comparing the G3 to the G5, but the G3 Mac to the G5 Mac. As a result, any newer systems will have many advantages over an older one.
In practical terms, though, it comes down to what you're going to use the system to do. My 400MHz G3 Lombard PowerBook works just fine for email, browsing, etc. However, it can't play a newer game to save its life, and it sucks at CPU-intensive stuff. But, honestly, surfing and email are essentially the same on my 400HMz G3 and my dual-2.0 G5.
Why do you ask?