We propose a power-efficient many-core server-on-chip system with 3D-stacked Wide I/O DRAM targeting cloud workloads in datacenters. The integration of 3D-stacked Wide I/O DRAM on top of a logic die increases available memory bandwidth by using dense and fast Through-Silicon Vias (TSVs) instead of off-chip IOs, enabling faster data transfers at much lower energy per bit. We demonstrate a methodology that includes full-system microarchitectural modeling and rapid virtual physical prototyping with emphasis on the thermal analysis. Our findings show that while executing CPU-centric benchmarks (e.g. SPECInt and Dhrystone), the temperature in the server-on-chip (logic+DRAM) is in the range of 175-200C at a power consumption of less than 20W, exceeding the reliable operating bounds without any cooling solutions, even with embedded cores. However, with real cloud workloads, the power density in the server-on-chip remains much below the temperatures reached by the CPU-centric workloads as a result of much lower power burnt by memory-intensive cloud workloads. We show that such a server-on-chip system is feasible with a low-cost passive heat sink eliminating the need for a high-cost active heat sink with an attached fan, creating an opportunity for overall cost and energy savings in datacenters.