The IEEE 802.11e MAC protocol specifies an enhanced distributed channel access (EDCA) mechanism with adjustable protocol parameters, providing differentiated access to wireless stations. The EDCA parameters are: CWmin and CWmax (minimum and maximum contention window), AIFS (arbitration interframe space), and TXOP (transmit opportunity). In this paper we study the joint setting of these parameters to maximize capacity and optimize performance under QoS constraints. We first revisit the analytical modeling in saturation and non-saturation conditions and provide more detailed approximations of the mean channel access delay and throughput. Considering two classes of wireless stations with higher and lower QoS demands, and optimizing with respect to average measures and constraints, we provide Pareto optimal pairs for the number of supported stations from the two classes for different parameter set configurations and load conditions. Further, we examine optimal parameter selection for a class of elastic traffic, in the presence of a delay-sensitive class whose parameters are fixed. Our findings show that drastic service differentiation can undermine capacity of a WLAN. They also demonstrate the optimality of jointly setting high values for TXOP and AIFS in order to maximize the throughput of the elastic traffic class while guaranteeing delay of the delay-sensitive class. We also reveal different optimal settings for CWmin for different load conditions. We summarize our findings as guidelines for the setting of 802.11e parameters in a scenario with data and real-time service classes.