Power is no absolute gauge of effectiveness. The speed of transmission will be throttled by congestion on the Internet. Even the fastest server will have some of its files slowed by erratic bandwidth speed. Here's a PC marketed as a web server. Some sites need this strength, but often you can well with less.
Reliability is important.
The machine itself usually runs continuously so computers with reputations for durability are wise choices. Servers that 5-10 connects per hour can run adequately on as little as 32 mb of RAM and with processors with clock speeds of 133 Mhz. An 8 gigabyte hard drive is widely regarded as a minimum. The Health Science Center's main server, which runs Windows NT with a 200 Mhz processor and 128 MB of RAM, serves about 250,000 files a day. Most new PCs today have no processor slower than 333 Mhz, and RAM has become cheap in the last three years.