My favorite "quiet" case is the Antec Solo. It is cheaper than the P18x cases and is not unnecessarily ginormous. The best thing is that it can sometimes be found for really cheap after rebate. I got one for $30 shipped after rebate! More typically, on sale (with or without rebate) for around $70-90 shipped.
The Solo has great ventilation, with 120mm Tri Cool fan in the rear (3 speeds) and unrestrictive grill. Front has a filter and is very unrestrictive, but is designed to not have a direct air path (to reduce noise). Front takes two 92mm fans (not included). The case also has dampening material stuck to the insides of the main panels.
The BEST thing about the Solo case, IMO, is the HDD mounting mechanisms. You have two options (and can mix/match to a limited degree). One is to mount four HDDs in trays that have really soft grommets. The other is to suspend up to three HDDs using the included bungee-cord-like mounts. This will completely decouple the HDD from the chassis. Why is this important? Well, once the rest of your system is quiet enough, the noisiest part of your system will be your HDDs. Decoupling them can drastically reduce the noise they make.
Here's my setup in a Solo with passively cooled CPU, chipset and GPU. This was back when I was running an E6750 @3.2GHz and the GPU is an 8800 GT with a factory OC.