After I change the file
Should I restart something?
No, changes to the hosts file are immediate. However, your browser cache settings may sometimes make it appear that changes you have made have not taken effect depending on what you are doing, so it's always best to clear your browser's cache if it appears a change did not take effect.
I've read that Windows XP service pack 2 or later doesn't use the hosts file unless dns client service is stopped. I'm not sure about vista or 7.
I've actually tried it and this seems to be true based on my limited experiments. I've not seen it actually documented anywhere so there may be more to it that that. If anyone has any knowledge of this or knows where to find something written about it I'd love to know more.