OGame takes place in a browser and similar to many other RTS games, however, if you want to try something similar yet different at the same time, try
Galaxy Online. It's a very popular game at the moment, and it plays similar to OGame. It does have a download, but it's uber small so I don't think it should be a problem.
I am currently looking for a more fantasy-orientated game like you requested, but go ahead and give GO a shot. I think you will like it.
Galaxy Online