AnandTech already published some of the first benchmarks of AMD's new quad-core Opteron processors and they've also taken the time to do some predictions on how the new architecture will perform on desktop PCs.
While the new Barcelona server processors seem to offer a good price/performance ratio there are doubts that AMD won't be able to replicate this on the desktop platform with its Phenom processors. AMD isn't sending out Phenom samples yet so AnandTech decided to mod a Barcelona server to turn it into a more desktop-like system. You can check out the results over here.
Anand compares the Barcelona with AMD's current K8 processors and says the K10 features about 15% more performance clock for clock. This seems to confirm the fears that AMD won't be able to punch Intel with the upcoming Phenom processors - although they might become competitive against Intel's offerings if these chips are priced cheap enough.