I upgraded to the PCP&C 750W Silencer from the GameXstream 700W when mine died, and it was a ton quieter. Despite everyone thinking that an 80mm fan will be louder, it allows for larger heatsinks and is actually quieter. My PC is pretty quiet, 6 Yate Loons in push/pull on my RAD running at ~1000RPM and I cant hear the PSU fan at all unless I put my ear next to it, even under heavy load (Crysis Warhead).
Of course you could always do what MacJunky said and just take appart the PSU are replace the fan with a different one. IIRC the one in there now is rated to ~70CFM and 35-40dbA at full speed, so you should try to get a fan that is similar air movement. I'd recommend the Zalman ZM-F3 as it is the best airflow/noise at any volume, and does just over 70CFM at max speed so it would be adequate to cool the PSU at full speed. Here is a link for it at NCIX:
NCIX.com - Buy Zalman ZM-F3 120MM Sleeve Bearing Case Fan Black 900/1800RPM 20/34DBA 3PIN Connector - ZM-F3 In Canada. (You can pricematch it to <$6 if you use shopbot/pricecanada)