Yeah, lets get back to the post ...
First, dual or quad cores are great for multitasking multiple single thread applications. You can easily burn DVDs or run the virus checker and play a game at the same time. Windows currently supports dual core but that doesnt mean that you cant get quad cores to work.
For multi-thread applications (and they are slowly coming out), you won't be limited to only dual cores but as many cores or physical CPUs you have. The application should be made to handle it.
Not all applications will be made alike nor will they have the same CPU requirements.
Whether dual or quad is better will depend on how you currently use your computer and also how the future will dictate our use.
If you can afford one then why not, but I wouldn't delay any purchase in anticipation otherwise you will never buy anything always waiting for the next new thing.