.NET for writing TCP servers requires that you use a multi-threaded approach. However this is easy to do so you should be able to achieve good performance on P4 class machines.
At the packet level it depends on whether your PDUs are ASCII or binary. If ASCII the stream functions make it really easy. If binary packets with a routing header, you will need to call the WSock2 functions for network byte order conversion.
To give you an example of performance, I developed a TCP server that takes a stream of PBX events from 1-500 clients (agents). After that a new instance of the Server is required; hence my server also load balances. The avg packet size is 60 bytes, largest is around 300 bytes. My server parses the packets in real-time and dispatches these to Web application clients. In essence, my server is more of a gateway. With that in mind I can tell you the simplicty of C# makes getting the framework of the application a snap.
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