Talk to Bobw, he has a 450 MHz G4 bumped to 1GHz, and likes it.
You could also check here:
http://promo.euro.apple.com/promo/refurb/de/
The US Apple Store has some good deals on the refurbished macs, so I would assume that the German one would too.
Also, I think $530 for a 450MHz G4 with 256MB of RAM and no display is a bit much, and I also agree that for what you're doing, the iBook will probably seem like a sports car compared to the G4, because there is solid evidence that the G3 and the G4 are close to equal in speed for the same MHz, as long as the application you are using isn't optimized for the G4.