Unlike other measurable quantities inside a computer (i.e. memory, hard drive space, front-side bus speed, etc), more cores don't guarantee a performance upgrade. This is because increasing core count doesn't mean the applications you run, including the operating system, are prepared to utilize them. Software needs to be made aware of, and written to take advantage of, the two extra cores. At the moment, chips with more than two cores are so new that software developers are still catching up with the technology. In practical terms: there just isn't that much software that can use four cores.
Does that mean that quad-core processors aren't the way to go? That depends, of course. If you're going to use your new computer for graphics editing, video editing and processing, or any other task that is processor-intensive and uses multi-threaded software, you might very well benefit from a quad-core processor. But if you're like most users, and your computer is going to be used for email, surfing the web, word processing and spreadsheets, the quad-core might just be overkill, and an unneeded expense.
Computer technology is heading the direction of quad-core processing, there's no doubt about it. As soon as the first multi-core processor was released to the public, it was only a matter of time until chip makers started trying to cram as many cores onto a single chip as they can. Right now, we're still in the early-adoption phase. Take a hard look at your use for the machine before you buy. If you can wait until the software catches up with the new technology, you might just save yourself quite a bit of money.