It depends on the app you are using. Programs that are multicore aware AND IF you have enough RAM will run all four cores at full speed will get more done quicker than dual cores.
My personal opinion is at this time 64-bit only has an advantage when you have over 4 GB of RAM and the apps you use need lots of RAM or you run lots of apps at the same time. Like Photoshop, After Effects, Vegas all open at once.
Only a few games at this time are multicore enabled so a faster dual core is better. On the other hand, many NLEs are multicore aware and can really speed up rendering with 4 cores. After Effects can also use 4 cores.
Personally, I start with the fastest quad-core CPU and work down until I find something I can afford.