Yes, the G4 Cube is 2X AGP at 100 MHz. I tried a couple of times to flash a PCI 6200 card for my B&W with Sonnet G4 500MHz. Both times, the card worked in the PC, flashed normally, but didn't work in the B&W, and never worked again. Apparently the B&W killed the cards I tried, an EVGA and an XFX. I thought it might be because the PC has 3.3V PCI slots and the B&W Mac has 5V PCI slots. The video cards were keyed for both voltages.

I haven't found OS9 drivers for NVidia. I use a couple of FX5200 PCI cards in the B&W, and they are limited to 256 colors and single resolution under OS 9, though they work fine under Tiger and support Quartz Extreme. They don't work under Jaguar and earlier.