Radeon 9500-9800, either Mac Edition or flashed with a Mac ROM. You'll be able to run all current OS X technologies with one of those. Avoid AGP cards for G5 machines as they use an AGP Pro slot.
Oh, you need a PCI card for a dual monitor setup? The best you'll do with PCI is probably a Radeon 9200 or if you can find a flashed nVidia FX5x00 or 6200. The 9500-9800 are AGP which will only work in an AGP slot, which the Quicksilver has one of.