I'd second Divinity's recommendation of Sapphire cards as a candidate for re-flasjhing. I have a Sapphire Radeon 9250 that works perfectly in my Powermac.

I've also heard of few problems with most 9700 and 9800 cards, but 9600 cards need to use the same memory as Apple 9600 (square not round chips).

Remember, most cards require more than just re-flashing. All AGP 8X cards require that you tape over or disable a couple of card edge pins. And the 9250 requires a surface mount resistor be moved.

All this can be intimidating work. I had no problem with my 9250, it took less than an hour. However, its easier to use an Apple card.