For an Internet site, decide how many peak concurrent users you need the server to handle, and multiply that number of users by the average request size on your site. Your average request may include multiple documents. If you are not sure, try using your home page and all of its associated subframes and graphics.
Next decide how long the average user will be willing to wait for a document, at peak utilization. Divide by that number of seconds. That’s the WAN bandwidth your server needs.
For example, to support a peak of 50 users with an average document size of 24 KB, and transferring each document in an average of 5 seconds, we need 240 KBs (1920 Kbit/s). So our site needs two T1 lines (each 1544 Kbit/s). This also allows some overhead for growth.
Your server’s network interface card should support more than the WAN it’s connected to. For example, if you have up to three T1 lines, you can get by with a 10BaseT interface. Up to a T3 line (45 Mbit/s), you can use 100BaseT. But if you have more than 50 Mbit/s of WAN bandwidth, consider configuring multiple 100BaseT interfaces, or look at Gigabit Ethernet technology.
For an intranet site, your network is unlikely to be a bottleneck. However, you can use the same calculations as above to decide.