This paper introduces a cost effective cache architecture called Enhanced Shared-Private Non-Uniform Cache Architecture (ESP-NUCA), which is suitable for high- performance Chip MultiProcessors (CMPs). This architecture enhances system stability by combining the ad- vantages of private and shared caches. Starting from a shared NUCA, ESP-NUCA introduces a low-cost mechanism to dynamically allocate private cache blocks closer to their owner processor. In this way, average on-chip access latency is reduced and inter-core interference minimized. ESP-NUCA synergistically integrates victims and replicas thus making it possible to take advantage of multiple- readers for shared data, and to maximize cache usage under unbalanced core utilization. This architecture leads to sta- ble behavior within the whole system across a broad spectrum of working scenarios. ESP-NUCA not only outperforms architectures with similar implementation costs such as private and shared caches by up to 20% and 40% respectively, but even outperforms much costlier architectures such as D-NUCA  by up to 28%, Adaptive Selective Replication  by up to 19%, and Cooperative Caching  by up to 15%. Moreover, performance variance throughout the set of benchmarks is 37% lower than with ASR, 87% lower than with D-NUCA, and 43% lower than with Cooperative Caching.