I realize this is low tech, but I have a large fan that I position just outside the grill of my computer. I leave it on low, and it seems to work pretty good. The sound is like white noise to me, kind of blends in so to speak with the ambient noise in the room. Also that same fan noise seems to cancel out some or all of the noise from outside my building (got wildlife including ducks, horses, the ocaisional farm machinery driving by) lol.
I run mine with the side panel off. In spite of defeating the "directed airflow" of the case design, each component runs the same or COOLER than with the panel on... didn't expect that. I suppose that if like you I ran an additional fan blowing into the side, things would cool down even further. You might try running yours with the fan but without the side panel, if applicable.
I buy the special no noise quiet fans... my head buzzes less at the end of the day... <img src="http://www.enflow.com/p.gif">
Or better yet, cool down the whole system: http://www.pugetsystems.com/submerged.php this one must be really quiet. Problem is, weight may become an issue...