High fabrication cost per bit and thermal issues are the main reasons that prevent architects from using 3D-DRAM alone as the main memory. In this paper, we address this issue by proposing a heterogeneous memory system that combines a DDRx DRAM with an emerging 3D hybrid memory cube (HMC) technology. Bandwidth and temperature management are the challenging issues for such a memory architecture. To address these challenges, first we introduce a memory page allocation policy for the heterogeneous memory system to maximize performance. Then, using the proposed policy, we introduce a temperature-aware algorithm that adaptively distributes the requested bandwidth between HMC and DDRx DRAM to reduce the thermal hotspot while maintaining high performance. The results show that the proposed memory page allocation policy can utilize the memory bandwidth close to 99% of the ideal bandwidth utilization. Moreover our temperate-aware bandwidth adaptation reduces the average steady-state temperature of the HMC hotspot across various workloads by 4.5°K while incurring 2.5% performance overhead.