I went with the 32mm Zalmans on my Mac edition 5500, and they seem to work pretty well... but I also have a 120mm Panaflo fan blowing down the length of the card. There's lots of airflow over the sinks, and it's still almost silent (all you can hear is a whisper of air movement).
I posted some pictures in
this thread, if you're interested.
I also considered using a pair of
copper Vantec Iceberq chipset coolers. I have one of these on a northbridge chip and it's quite effective, but with the 5000 rpm fan it's relatively noisy. I suspect they would seriously outperform the stock coolers at similar noise levels, though, and they wouldn't overflow into a second PCI slot. They also look damn sexy.
Put some of
these babies on the RAM, and you'd have the nicest-looking 5500 ever.
Of course, all that would set you back about £50...