This doesn't answer your exact question, but I successfully watercooled a 5800 with a Koolance LH-06 block. It's certainly quieter (0dBA +/- 3DdB), but it's not a fan.
I bought Walter's 5800 Ultra
for more NV30 watercooling experiments. I'm concerned that 'normal' RAMsinks aren't going to be sufficient for the DDR2 at 500/1000, and waiting for the Cool Matic FX RAM blocks
to get back in stock. (I left the stock RAMsinks on the 5800 non-ultra so I wouldn't have to worry about DDRII for that experiment.)