could you pelase tell me what's the difference (i never heard of lighttpd before) ? pro / cons ?
and what would you pick for a website who have to deal with a lot of querys (like 20,000 min per day) ?
It really depends what you want to do and what resources you have. But unless you have a very heavy traffic (which you do not have, 20k/day is fairly light) or very complex/dynamic pages that require a lot of server processing, just pick a server with the required features. Apache is a safe bet, in my opinion, and it is not slow, as it is often implied.
Just as a reference, one web site I manage runs apache on an old dual CPU/2G ram and serves 1+ M files per day w/o breaking a sweat (mostly images -- dynamic pages run on app server).
As long as you don't need any plugins which are only available for apache, lighttpd is usually a better choice when performance is an issue.