The spec's for the S-Flex fans are here: http://www.coolerguys.com/sff21.html so you can compare them to the slipstreams. To get the same amount of airflow with an S-flex, it looks like they would be louder than the slipstream, though they apparently last a whole lot longer (150,000 hours for the S-flex vs. 30,000 hours for the slipstream).
As far as how much airflow you need, it depends on how hot your receiver runs and how much room there is around the fans. If you've got an inch between the fans and the wall, they're not going to put out nearly as much airflow as if you've got six inches of space.
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