changes to network confiduration do not take effect until the network is rebooted.
Interesting, sometimes you can activate the network connection without the reboot:
Assuming you set your IP address is 192.168.1.1, your subnet mask is 255.255.255.0,
and your default route to the internet is 192.168.1.2, type as root:
# ifconfig eth0 down (if the network is up)
# route del default (remove the default routes)
# ifconfig eth0 192.168.1.1 netmask 255.255.255.0 up
# route add default gw 192.168.1.2
Then try the pinging and webpages etc.
Otherwise if it doesn't work, reboot.