Hey Hassan, I have found this interesting proxy setup in youtube: http://www.youtube.com/watch?v=S65Gp79YHu8
Which is exactly what you need for your case. I also see now that mikrotik routers do make it very simple to setup.Note that this setup uses a upstream proxy which is using port 8080 and you will need to define a rule (maybe on the mac address level) which will not redirect traffic from the proxy towards the clients and the web.
Eliezer
