hardening wordpress for noobz

we are using wordpress multisite as a CMS and have taken the plunge by running our own server (ubuntu/bitnami install) using amazon. my question is what are some of the best things to do to deal with all the nasty bots out their trying to slow down our server. we don’t enable comments (right now) and have tried a few plugins to help us find some of the nasty stuff coming to say hi, but we are looking for some advice from people who know what they are doing. for us speed is almost as important as security so we don’t want to do something dopey that slows down the site. any suggestions, articles or plugins would be greatly valued. thanks in advance.