As far as I'm aware even in the newer games the multi-core support is rather limited right now and most of them are primarily limited by the graphics processor anyway. A faster dual core will generally be more useful in practice than a slower quad, unless you're heavily into image processing or movie rendering.
I agree... faster dual core is better than slower quad. Unless you're transcoding 4 DVDs at a time.
If anything is multi-threaded right now, it'll work best with a dual core. In the mean time, a program will only use one of the cores, which is lower clock speed on the quad you listed, thus lower performance.