While the ThunderX isn't yet shipping, it's promising 48 ARMv8 (64-bit) cores at 2.5GHz while still being a "low-power" SoC. There's also to be cut-down versions of the ThunderX in a 24 core configuration and support for dual socket configurations using a "Cavium Coherent Processor Interconnect." DDR3 and DDR4 memory are to be supported.
Announced thus far are ThunderX_CP, ThunderX_ST, ThunderX_SC, and ThunderX_NT varieties. Various Linux distributions will be supported for the Cavium ThunderX such as Fedora, Ubuntu, openSUSE, and MontaVista. Cavium is expecting for general sampling of the ThunderX SoCs in the last quarter of this year.
More information on this promising ARMv8 server SoC can be found via the Cavium press release. For those that think this sounds a bit familiar to the now-defunct Calxeda with their former ARM server ambitions, there are some former Calxeda employees at Cavium.