I'm using an app crashdamage told me about called RAM Truth. In the available RAM section I only lost a tiny 5 MB with nova installed. As I said, it wasn't Nova that was using much RAM though, it was the other theming apps I was using with it.
You can't actually "gain" RAM using a 3rd party launcher, that to my knowledge is physically impossible. But what nova cleverly does is run as a cached process so it doesn't run in active RAM by default. That's why you can run it over top the stock launcher without any performance hit. And nova also slightly increases app launch and scroll animation speeds by default in its settings.
What Nova doesn't do is actually make your phone itself run faster. By that I mean for example if you're in your browser or playing a game etc, Nova has no control over how fast or smooth your apps are behaving aside from just increasing the launch animation when you first run them.