In a mildly surprising move, AMD has revealed that AM2 will accept AM2 or AM3 CPU packages.
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The most recent AMD roadmap is also very clear to state AM3 "supports either DDR2 SDRAM or DDR3 SDRAM, but not on the same motherboard." This is great news for upgraders, as there is a very clear upgrade path: CPU, motherboard, and then memory. This also infers that AM3 CPUs will have both DDR2 and DDR3 support on the integrated memory controller.