Which is faster?

Are you running OSX?

A dual 500 Mhz G4 will run much faster if you got more than one app using CPU cycles.
ie) Surfing web while iTunes is encoding.

Otherwise for single tasks, the dual 500 Mhz will slightly edge out the 733 depending on if your software is multi-threaded. Most are.

 

I'm pretty sure the dual will be faster in most tasks.

 

yep, especially when dual processers have very well matured for apple computers. very supported, etc so it's very worth

 

That depends on what you are running. In general the Dual 500 will be faster, but some applications will be capped by the 500MHz and not really make use of the second processor.

However, remember if you have multiple appplictaions open and running (mail, IM client, iTunes, web browser) The dual 500 will be better.

 

I agree with Bear. In generall, I'd go with the dual.

 

As others have said, the dual 500MHz G4's should be faster for you.