The best cards you can currently get fanless are the Sapphire RADEON 9800Pro/XT "Ultimate" edition cards -- but I'm not sure if these will fit in your Shuttle (at the very least, it'll eat up your PCI slot). You can also buy the same cooler (the Zalman ZM80C-HP, or the new ZM80D) and mount it on most video cards, though it voids the warranty on the card to do so (and, again, it might not fit). I don't know if this will fit on the next-gen cards (X800, 6800).
The fastest fanless 1-slot card I know of is the RADEON 9600. Slower than the 9800Pro by quite a bit, though.