The Alpha PAL8045 and Swiftech MCX462 have nice square shapes for ducting and they are very large. You should get an aluminum duct and duct it to your case exhaust. Only use a fan on the case exhaust. If you make the duct tight enough around the heatsink (I think a rubber band would work), then it should act as an extension of the heatsink. Although it's not that efficient of a transfer, it does help a bit. PM bluefront for more information.
If you don't want to go through that trouble, a Thermalright SLK-900u or SLK-800(a or u) will do fine with any quiet fan (Panaflo L1A).