There isn't really a way to tell unless you give us more information. It depends not so much on the processor but on the RAM, the type of HDD's, and the type of content being served.
I still haven't got the server yet. It's likely that it will be SCSI H.D, 667 / 1066 Mhz RAM, the content is html web serving put also php, forums, image galleries, copuple of MySQL databases... It will be connected to 5 IPs. (not sure if it's possible to make one IP a nameserver to the same server, any ideas????)
I agree, it's really not so much about the hardware but rather how good you are at optimizing the server as an overall unit (e.g. all components, suitable fs, tuned apache/lighttpd, and the applications running on it).
You can have a perfectly good web server capable of handling hundereds of concurrent connections, but if your applications and database are put together badly then there's really no hope. So at the end of the day it really boils down to optimizing the application and tuning the server to your applications resource usage patterns.
At the moment my most favoured hardware for web-serving and just about all data-transfer-intensive applications has to be the Sun T2000, it's a bit pricy to start with, but it'll pay off in the long term.