Two years ago, people will be dreaming of two cpus in one casing. Utilising two cores would speed up applications such as adobe photoshop and also video rendering process. Knuth suggested that there is no such need for quadcores as most of the time, not all four cores will be utilised if the user is only going to use the pc to run Microsoft Office applications and serve the internet. In such a situation, it will be a waste for purchasing a quadcore pc.
In my personal opinion, having an up to date pc is a matter of choice. One might not know whether there will be such a need to use more cpu intensive softwares. One good example will be the use of adobe photoshop CS3. If you only have 1GB of ddr2 667 memory with a single core cpu, I think it will most probably take much longer time to load, not even talking about accessing the pictures to be digitally enhanced than a quadcore with the same amount of system memory.
I still feel that a quadcore pc is future proof and quadcore will be the ceiling for any upgrades. Intel has come up with eight cores with its Skulltrail platform but it is just too expensive to be available commercially. I think quadcore pcs are sufficient and it is useful if you are using it to open many windows as I do and multitask.