Data centers frequently face significant memory under-utilization due to factors such as infrastructure overprovisioning, inefficient workload scheduling, and limited server configurations. This paper introduces Agile-DRAM, a novel DRAM architecture that addresses this issue by flexibly converting the under-utilized memory capacity into enhanced latency performance and reduced power consumption. Through minor modifications to the conventional DRAM architecture, Agile-DRAM supports multiple operational modes: low-latency, lowpower, and the default max-capacity mode. Notably, Agile-DRAM facilitates agile transitions between these modes in response to workload fluctuations in data centers at runtime. Evaluation results demonstrate that the low-latency mode can boost singlecore execution speed by up to 25.8% and diminish energy usage by up to 22.4%. Similarly, the low-power mode can reduce DRAM standby and self-refresh power by 31.6% and 85.7%, respectively.