Core 2 is stealing a lot of thunder from the X2 and Opteron, leaving AMD in a sticky situation about where to go next. Wanting some of that thunder back, they are now talking about their upcoming Quad Core CPU, based on the “Barcelona” core. Not truly a CPU with four cores, it accomplishes four the same way Intel's first-generation “dual-core” CPUs did with two, and that is placing two dual-core CPUs in the same package, side-by-side. It's a bit enhanced from that, and each CPU will be able to share an additional L3 cache, giving the CPUs a boost to performance. Starting at 2MB per physical unit, it will be shared by all four cores and will come on both the Athlons and Opterons. Neat stuff. The individual cores will carry the typical 64KB and 512KB L1 and L2 caches, respectively, possibly expanding in the future to 1MB for L2.
For power management, Barcelona apparently won't have any problem with that much horsepower in such a small space, with a very complex system of voltage and clock rate control above and beyond just stepping the CPU down that can individually remove power from components when they aren't needed. How long until we see it, though? Hopefully soon.