By including dedicated processors, NICs designed specifically for use in servers should improve performance. However, what network operating system you're running has a big impact what benefits you'll really get. However, there are cards available that are designed for use specifically within servers. They typically cost more than the cards reviewed here, but are meant to have additional benefits.
Fault tolerance is critical for any server that has to support a large numbers of users.
While a server can have redundant power supplies, disk drives and other hardware, if you only have one network card in the server you're introducing a single point of failure. By having two or more network adapters, you should be able to introduce a level of fault tolerance. This can be managed either by the adapter driver or the network operating system. From a performance perspective, the reality of the situation depends on which network operating system you're using. Tests performed in PC Magazine Labs earlier this year show that under Windows NT, server NICs produce little benefit over standard adapters.