if you have a lot of background programs running, you may notice a performance increase on games etc with a quad, however i only know of 1 games written for a quad and that has just taken a backstep for the time being. its like when dual core came out it took a while for programs to be written for them. common programs are written for the most popular (average) system so most people can use them to full effect.
i have an amd 6400x2 @ 3.6hz system and a QX6850 64 bit system they both play everything at full settings with little difference as programs can only be taken so far.
the quad will be more future proof at the moment i think there's little difference depending how much background crap you have running.