Obi wan my understanding from what information i have gathered on this subject is....
The Voodoo5 5500 AGP will only operate at the 1x (66MHz) transfer rate, although the 2x transfer is selectable in '3dfx tools' it actually doesn't work. The motherboard will automatically default back to PCI66 mode but the 3dfx tools option will remain at 2x.
The reason the Voodoo5 cards will not operate at 2X AGP is because the AGP interface requires an AGP Master Chip. Because the board is dual-chipped, it cannot assign a master chip, and so defaults into AGP 1X (really PCI66 mode). Thus the Voodoo5 5500 AGP will only run at the 1x transfer rate which is not really AGP but 'double speed PCI'. The way to overcome this would have been to include an AGP bridge controller chip onboard, like the Voodoo5 6000.
There are rare Voodoo5 5000/5500 engineering samples which are mounted on a 1.5V/3.3V AGP Universal 2.0 PCB. This does not mean the card will run in 2x or 4x AGP, it will not, the card will install in some newer AGP 2.0/AGP 3.0 motherboards, which is the only advantage, but will still operate as a PCI66 device. There is nothing that can be done about this.
As to the agp x4 v5 6k im not sure why but it might be a power issue?