@Andreas_Wolf
opsss :) corrected :)

@Jim
Adreas has told you all about AGP :)
here some simple exemples

1) ATI 9800 XT OK for 2x AGP slots and also for 4x AGP slots
http://themacelite.wikidot.com/local--files/flasher-s-buying-guide-9800/9800pro.jpg

2) ATI 9800 XT OK only for 4x AGP slots
http://themacelite.wikidot.com/local--files/flasher-s-buying-guide-9800/9800xt.jpg

3) ATI HD AGP are OK only for 4x AGP slots, but It's needed a flash ROM Macintosh.