It's an ongoing conundrum: do I upgrade now or wait till the next platform comes along?
The problem is, if you're continually waiting for the next thing to arrive, you never actually purchase anything! The next big platform on the horizon is Intel's Santa Rosa which promises enhanced wireless (Draft-N), "Intel Dynamic Acceleration" for better Vista performance, and a new Socket P.
Is it worth waiting for? Maybe. Like every new platform, it is bound to be more expensive than the current generation and have its share of bugs. But what will be interesting is how it will affect the prices of current stock. If you're looking for a bargain, Santa Rosa, and its business counterpart Centrino Pro will drive down the price of existing Core 2 Duos when they're released sometime in Q2 2007.
If you need a notebook or desktop now, there's never usually any reason to wait. Just read as much as you can on models you are interested in -- forewarned is forearmed. And while prices will fall on any technology you invest in, if you do your homework you can be assured of getting the best deal at the time you buy. As for right now, the existing Core 2 Duo family is unlikely to evaporate for the next 18 months, so you're assured of an upgrade path if you need one.