There are SOME restrictions depending on MotherBoard and it's AGP slot.
If memory serves there are two types of card and 3 types of AGP slot.
1 takes AGP type A (for example)
1 takes AGP type B
1 takes AGP A & B
now, the trouble is I can't remember the difference or why they are different!
It may well be because of the version whether is AGP, AGP 2x, AGP 4x or AGP 8x.
Sorry I can't be of more help until I find the article I need.