Previous Power5 articles hinted at a four-fold increase in performance over the current Power4. It appears that IBM will be accomplishing this with Multithreading according to this Silicon Strategies article.
IBM launched the Power4, the first dual-core server CPU, in 2001. With the Power5, it now hopes to be the first to blend both multi-core and multithreading technologies.
According to the article, the chip will debut at 1.5GHz at a 130-nm process and "it appears to the operating system that there are four CPUs on each chip."
The Power4 and Power5 chips are IBM's server CPUs. A derivative Power4 chip, called the PowerPC 970 is rumored to be used in PowerMacs as early as this year. There has only been minor hints at subsequent Power5 derivative chips.
But The Inquirer claims that it will come to pass, giving the derivative chip the advantage of multithreading:
This will come as interesting news to Apple fanatics too. IBM is producing a cut down version of the Power5 with just a single core. However, that single core should be capable of multithreading, another boost for Apple quite apart from the advantages of going to 64bit.