I'll also mention www.etymonline.com. I use it quite frequently. Dictionary.com actually gives pretty decent etymological info, but etymonline is a bit more thorough. Helps me figure out if a certain meaning of a word was used at a certain time. Often, it wasn't.

LC Blackwell, that's one reason I think it's important to look at modern scholarship on your topic. You need to know what people have discovered recently. Older sources and even primary sources might be proved wrong. Also, how we look at history has changed massively, and you don't want to be presenting the history through a lens that is outdated.