Digit-Life just posted an article where they compare quad-core processors with dual-core ones. The contenders include only two relatively new processors - Intel Core 2 Duo E7200 on Wolfdale core and Intel Core 2 Quad Q9300 on Yorkfield core. Here's a bit:

The new core is apparently much faster than the old one. The main 'sufferers' here are old quad-core processors (Kentsfield): very few applications can use four cores; besides, dual-core processors have been overhauled so that a dual-core Wolfdale performs on a par with an equally-clocked quad-core Kentsfield (3% of performance difference for more than a 1.5-fold difference in their prices.) However, two groups of users don't have to worry about their investments: those who use 3D Modeling applications and Adobe Photoshop. Quad-core processors are almost always faster than dual-core products here.

The final chart shows no radical changes: Core 2 Duo processors (especially Wolfdale) are still the most reasonable choice in this family. Quad-core processors make sense only for certain applications - you must be sure that your application can effectively use all four cores. Even attractive prices for the cheapest models will hardly be a ponderable argument for quad-core processors, if you are not sure that performance in your application of choice will come up to your expectations.

If you didn't test your tasks for performance gains on quad-core processors versus dual-core products, or you didn't obtain definitely positive results - you will most likely fail to get significant performance gains from using a quad-core processor instead of a dual-core model these days. That's the main tendency now. There are only a few exceptions. Advantages of quad-core processors are open to those users, who know for sure what they want from a CPU and how their applications of choice can use it.