Guys... here's what I've seen on a forum that's arguably the size of this one.
5% of the people contribute 95% of the content.
It's the CONTENT that makes a forum worth coming back to. The 95% that do not contribute (through advice or conversation) are only worth one thing... advertising fodder.
We drop the banner ads for all that contribute (any amount). We also drop banner ads for those who contribute materially (through participation). For the rest, the advice and good reading is free, but they get to look at the advertising. It's VERY simple to manage in VBulletin. It takes just moments to ad the modules that take contributions online and do all the work for you.
Google AdSense is a great resource Keith, as are many targeted marketing portals. They will not affect your contributing membership at all, or even be seen.
I know you have an idea of what you feel this forum should generate in income (above expenses), and that PERFECTLY OK. You shouldn't share that with anyone except those in your business plan, and make the steps necessary to grow this business into a self-sustaining model.