Maybe I should have been more specific with my post...
I have a x600 that runs (albeit hot) fanless with a small aluminum heatsink, all factory installed. The card has been running fine for the last few years. Here is a picture of the card I have for reference: http://www.txcesssurplus.com/servlet/the-4519/ATI-Radeon-X600-PCI-dsh-e/Detail
I suggested the heatsink in my previous post for two reasons; First it was only $10usd where as many aftermarket HSFs for GPUs are quite expensive. Secondly, the heatsink is huge compared to the stock heatsink on my x600, and has heatpipes. I therefor assumed if installed correctly that the heatsink would be able to perform well with a low power card like a x550. The fan installation isn't too simple but if you did install it I think you would find the fan to be almost silent on the low setting. I just turned the fan to low on mine and my overclocked 8800GT runs at 60C while running Folding@home.
One warning, if you do purchase this HSF and plan on installing the fan, know that it takes up an additional PCI slot space below the GPU.