This paper studies the impact of network interface cards (NICs) on network server performance, testing six different Gigabit Ethernet NICs. Even with all other hardware and software configurations unchanged, a network service running on a PC-based server can achieve up to 150\\% more throughput when using the most effective NIC instead of the least effective one. This paper proposes a microbenchmark suite that isolates the micro-level behaviors of each NIC that shape these performance effects and relates these behaviors back to application performance. Unlike previous networking microbenchmark suites, the new suite focuses only on performance rather than aiming to achieve portability. This choice allows tight integration with the operating system, eliminating nearly all operating system overheads outside of the device driver for the network interface. The results show that the throughputs achieved by both a web server application and a software router have an evident relationship with the microbenchmarks related to handling bidirectional streams and small frames, but not with sends or receives of large frames. This paper is also available as Rice University Technical Report EE0401.