At the moment, DDR3 offers little performance benefit in exchange for a LOT more money. And even then, only at the highest clock speeds. So for building a system *today*, there is little to be gained in the exchange except for bragging rights. About teh most you can say for sure is the Cost~Is~No~Object Benchmarking crowd will find worth in obtaining the last few bunghole-points on a graph. So if your ePeen responds enough that paying 3x the money for RAM that doesn't perform much better than what you have now makes it worthwhile...
Given the pace of development, a year from now the story will likely be different. I'd presume that Nehalem should make good use of the stuff once Intel is done cooking it. But for building a system today, the answer is "No, don't buy DDR3". The money can be much better spent elsewhere.