HotHardware.com evaluate the features and performance of AMD's new 65nm Brisbane core based Athlon 64 X2 5000+ processor. Here's an excerpt:

From what we've seen in our testing, AMD delivers on their promise when it comes to a lower power CPU. In regards to both wattage draw and thermals, the new 65nm CPU was better overall than the same speed processor based on the older 90nm "Windsor" core.

Lower power also comes with slightly lower performance in the case of AMD's 65nm Brisbane core, however. In synthetic testing we saw the widest performance deltas, some of which hovered around 14.5% compared to the older CPU. Yet, when we shifted to real-world testing, the margins were a more marginal 1-2.5%. In the end though, slower obviously isn't better. However, in a side-by-side comparison, we think users would be hard pressed to actually "feel" the difference between the two processors. In fact, during our evaluation of the 65nm AMD Athlon X2 5000+, we ran the CPU in a normal work environment for several days and then switched it out with its 90nm counterpart and there was no obvious difference is day to day performance. In addition to this, the 65nm Brisbane core-based processor overclocked much better than the Windsor-core. And the higher overclock would easily offset the higher latencies. If you're in the market for a mid-range Athlon 64 X2 processor and plan to do some overclocking, look to the newer 65nm processors. Otherwise, if you're planning to run an AM2 system in an all-stock configuration one of AMD's 90nm processors offer slightly better performance.