AnandTech has put together an 11 page article, bringing together all that is known about Barcelona.  They take you through the chip, from what native quad core means, to the doubling of the stack used for indirect branch predictions.

There have been a lot of changes to the old K7 core, so take a few minutes and read through this article.
AMD had worked on dramatic successors to the K8, rumored to be K9 and K10, but both appeared to be scrapped or at least focus was shifted away from them in favor of a more evolutionary take on the K8 architecture. The main difference here that allowed Intel to catch up to AMD’s performance is that while Intel’s Pentium 4 team was operating on the usual schedule of a 5-year micro-architecture cycle, the Pentium M team at IDC was updating its architecture every year. Banias, Dothan, Yonah and Merom/Conroe all happened in a period of four years, and during that same time AMD’s K8 remained unchanged.

If Intel had continued down the Pentium 4/NetBurst route, sticking to the usual 5-year design cycle would have probably worked just fine for AMD but Intel had the luxury of having two major micro-processor teams working in parallel, one of which had a much better idea. Luckily it would seem that AMD realized it needed to compete with Intel using smaller evolutionary steps every couple of years rather than leaving an architecture relatively untouched for 4 – 5 years and thus the Barcelona project was created. Although it’s set to debut around a year after Intel’s Core 2 Duo that swiped the performance crown, Barcelona is AMD’s best chance at remaining competitive.