It depends on what you want to use the PC for.
If you are looking to get maximum performance in current games you can get a faster clocked dual core (X2) for the same price as a slower-clocked quad core (X4). The clock speed will be a little better for running a single intensive application.
If you intend to multitask, or to keep the processor for a long time a quad core will be better for multitasking and will age better. (as you may imagine--more cores do more things).