If you're simply going for raw, number-crunching power, then yes, you can do much better than a dual-processor G4 upgrade with $640.
Owning and using a Macintosh, for some people, is more than just "lookie how fast my machine can go!" It's like an old car you get attached to and don't want to let go no matter how slow it goes. If you really love that car and have it "set up" in some way that just fits you perfectly, sometimes it's worth rebuilding the engine for $2000 instead of putting that money toward a new car, if only because you don't wanna get rid of the car. Sure, a new car would look better, go faster and be cleaner, but it wouldn't be your old, trusted, friendly car.
To some Mac users, using their computers is more than just raw speed.
I see you don't have a Macintosh listed in your signature. People tell me I should get rid of my VW with 200,000 miles on it because I spend as much money keeping it running as I could on a payment on a new car. I'm not getting rid of it just yet -- it fits me perfectly, as no new car could.
There are also idiots running amok spending $50,000 souping-up 4-cylinder $20,000 Mitsubishi cars, when they could be spending that money on a Porsche, which would be much faster.
"Fastest" doesn't always mean "best choice," depending on what your needs are. Not everyone is looking to get the fastest thing on the market for their money -- the user experience, simplicity, quality and many other factors weigh just as much when someone is spending that amount of money.
