I just switched from a old garbage case with 3 - 80mm Thermaltake Smartfan IIs in the case, and one 80mm Thermaltake Smartfan II on a Thermaltake Volcano 9 for the CPU cooler. The case fans were running auto based on temperature, and I had the CPU fan manual at about 3700 RPM. Needless to say, this setup was loud and ran hot for Winter room temps (CPU temps up to 50 deg C) with the case cracked open.
I bought an Antec P160 and a Thermalright SLK-947U cooler. The airflow with one 120mm fan in the case seems to be awesome. My video card temps are down, and case temps are also down. Also, what a difference the SLK-947U makes, even with a Smartfan II at 3000 RPM. This heatsink is keeping the CPU well within 10 deg C of the case temps. Since this is an aluminum case, I put some thin insulating foam (from Lowes) on the side panels to kill any resonance. I like the look of this case versus the other Antec 120mm fan cases.
I am running a Athlon 2500+ at 205 MHZ FSB, Asus A7N8X Deluxe, 2 HDDs, BFG 5900 at 400/850, and an Antec True 480 powersupply. The 120mm fan is connected to the FAN power connector on the Antec powersupply.
With the new case, this rig is much, much quieter. Even after gaming or Prime runs, the fan noise remains pretty quiet. CPU temps remain below 40 deg C with 23-24 deg C room temps. I bought a second fan for the case, but I don't think I need it. The only change now would be a 92mm fan for the SLK running at lower RPMs than the Smartfan II.
To tell you how quiet this is, I have a work laptop (Dell C-600) on the desk, and the fan in the laptop is much louder than the Antec rig. I imagine you could pretty much buy any of the Antec 120mm cases and be happy with the sound results. After reading about the possible resonance problem, a simple fix kept that from being a problem with mine.
Good luck. Believe me, spend the extra few $$ and get the SLK. It will give you more flexibility in keeping the case quiet. The removeable motherboard tray in the P160 makes mounting the SLK a lot easier. One of the reasons I got the Volcano was that my old case did not have a removable tray.