As Mike suggests, segmenting and using ICS (Internet Connection Sharing) are two reasons why one might want multiple NICs (Network Interface Cards). Other reasons include network bridging, NIC teaming, and load balancing. Search on those terms and you'll find out more about them.
Personally I would rather accomplish segmenting and/or ICS via routers rather than PCs, and the ability to team/bond or load balance using conventional networks is more intended for servers, not clients. Bridging can be useful in many situations, but for home or small business networks it is perhaps most useful for connecting networks using different types of media (i.e. connecting two wired networks together via a bridged wireless connection): http://www.microsoft.com/windowsxp/usin ... ril22.mspx
Bottom line: Multiple NICs are not something I get very excited about, my guess is the motherboard mfrs. find that multiple NICs are a feature they can easily add to high-end motherboards to make them more "spec (as in specification) -tacular". <g> Scott.