Yesterday http://wired.com launched the idea that as MS chose IBM to make the CPU for the 'Xbox Next' chances were high it would be a cpu based on the PowerPC 970 (similar to the G5 processor that powers Apple's developed jointly by IBM, Apple and Motorola).

More articles came out today with other possibilities. From newsfactor.com:

Gartner analyst Martin Reynolds questioned whether the IBM supplied chip would be PowerPC architecture. Much of the Xbox software is based on Windows code, he said. "If Microsoft goes away from an x86 processor, they're going to have to maintain a separate code base, and that just doesn't make any sense," he told NewsFactor. The IBM Xbox processor actually might be an x86 processor, he said. "IBM could get that license from AMD," he pointed out. "We know that IBM and AMD have a relationship, so it's possible that IBM could build the AMD architecture and deliver it in the Xbox package." As a possible alternative, IBM may have an opportunity to integrate the functions performed by more than one of the Xbox's chips into a single chip, providing significant cost cutting, Reynolds said.

IBM will not actually need to rewrite the code for a PowerPC chip, but merely recompile it, Aberdeen analyst Russ Craig said. "My guess is that there's probably a fairly easy switch." Craig speculated that IBM will supply Microsoft with the same processor it supplies to Nintendo, which is the PowerPC. Additionally, since IBM is developing an ultra-fast processor for Sony's Playstation, "it could be that IBM has done some tweaks to an architecture that may be better for game machines than a Pentium."

To ensure backward compatibility, Microsoft reportedly will use the Virtual PC technology it acquired from Connectix last year, although using Virtual PC technology would be a performance detriment, Reynolds said.

Read more about it on newsfactor.com and also check out the article on theregister.co.uk