Dynamic voltage scaling (DVS) and dynamic power management (DPM) are widely-used techniques to reduce energy consumption in modern computing systems. Although combining these techniques can save more energy, there has not been much work focused on energy-optimal combination of these techniques under variable memory clock frequencies. In this paper, we explore system-wide energy-optimal frequency space for a memory-aware DVS technique based on a stochastic memory access model for systems with frequency-variable memory devices. In addition, we propose a simple but practical DVS method, which can be applied to an actual platform.