Can you browse by numerical address? Try
http://64.179.4.146 If that works, check /etc/resolv.conf . If you're not running a nameserver (like bind) /etc/resolv.conf should contain the word nameserver and the numerical address of your ISP's domain nameserve. If you call pppd with the option usepeerdns it will create a resolv.conf in /etc/ppp/resolv.conf when it connects with your ISP