The ATI Radeon 9800 Retail card, just announced today, is actually AGP 2x,4x. The Radeon 9800 Pro (OEM from Apple, with new G5) is 8x only (so it wouldn't even work in your box).
The only downside for you might be that the retail version doesn't have ADC. But on the upside, you get a sweet suite of controls (finally) for the card (i.e. new confiuration utility for the 9800). Even better, the card is (allegedly) available as I type from the ATI.com store.
/Cincinnatus