Have you guys used "ubersuggest" at all? I've found it can be a great little tool for some new keyword ideas. It doesn't always come off and can depend on your niche but its given me a few great keywords that convert really well.
I also think, using ppc performance is a great way to check on if a keyword is worth opting for. As long as the landing page is the same, you can check conversion rates of that Keyword in PPC and it gives you a better insight into whether or not to build for that specific term/terms.
All going well, you could cut spend on PPC if you start to rank in the top positions, but that's up to you. I suppose as long as its making profit, you may as well take as much real estate on the page as possible.
Hope that helps, I've not used word stream yet but will give it a go, thanks for the heads up.