www.quietpc.com has some nice (though not real cheap) acoustic isolation foam that you can apply to the inside of the case side panels, top and bottom to quiet things further.
Directron.com sells some all-rubber fan mouting devices (no metal screws) to help reduce fan vibration from being transmitted to the case frame. They also have a large rubber power supply washer that also prevents the metal-to-metal vibration of the PS to the case. Who knows how much they
really help, but they did reduce the pitch of the case's fan noise, making it seem quieter.
To quiet my case (a rather big Directron SF201), I've added the above items, a Zalman CNPS6000-Cu CPU cooler and replaced the included 92mm Zalman fan with an even quieter 120mm Vantec. I disconnected the case's stock two 80mm front intake fans (loud!)and replaced the stock rear fans with two Vantec Stealth 80mm case exaust fans running off a Zalman fanmate (at about 50% of standard 12v speed). For air intake, I added a Vantec Stealth 92mm side intake fan blowing onto the motherboard & passive NB cooler. It's not yet "Dell quiet," but it's pretty darn close.
Next item eventually will be a quieter power supply than my Enermax (which really isn't that loud). If after all is said and done, and it's still making more noise than I'd like, I'll try one of those passive coolers for my Radeon 9800 pro. I just don't want to do that much surgery to the card if I can avoid it.
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