Core 2 Duo. The architecture is the best out there at the moment. That and they're more energy efficient. Although if you're willing to wait till the 45nm Core 2 Duos and Core 2 Quads come out ($300), you will gain an additional 40% increase in performance with the Penryn series. Which in my opinion is a by far better purchase-minded choice. As buying into a Core 2 Extreme right now, would be unwise when 40% faster CPUs are coming out by the end of this year.
IN addition to that, new GPUs are coming out, which will lower the cost of the 8800 series product you are considering to purchase. Budget-wise, wait to buy. Not only for purchase, but so that the money you can save later on can also be used to upgrade to other products, such as faster RAM, or DVD drives or Hard Drives. Any performance gain is always welcomed.