One of the best tuning tricks for Unix servers involves increasing the size of the listening queues for incoming TCP connections. This comes about due to the long delays that are inherent with modem-based connections; due to the amount of time it takes your server to service a request, it needs to maintain a longer queue length to hold onto the other incoming requests that must be responded to.

Programs such as swap, sar, ps, and top can help you identify the memory usage requirements for your OS and applications. you must minimize the paging of memory to disk, since server performance will suffer when paged memory is used.

In addition to application and OS memory usage, make sure that you retain sufficient memory for network usage. Your servers are very dependent upon memory to be used as network buffers, which are responsible for the receipt and delivery of data to your server. Network buffer requirements can be identified by running netstat -m. Pay close attention to any indications of memory requests being denied or delayed, which would indicate insufficient memory. It may be more efficient to decrease the size of the buffers servicing requests, as much of the HTTP traffic normally found at Web sites is considerably smaller than typical default buffer sizes.

Another network memory issue that merits notice is the amount of memory used for reverse DNS lookups when logging host names. You can save a considerable amount of memory and network usage by turning off DNS lookups, and possibly reassigning it to another machine that processes the log files offline and does the reverse resolution at that time.

you will get more info from:
http://www.psc.edu/networking/perf_tune.html