I will add a bit of advice here as well. Some people see a lower ranking so they start to panic and they go changing variables. What I mean is this scenario:
Uhoh, my google ranking is down. I better change something. Hmm, let's try search engine friendly URLs. 2 weeks later, it hasn't improved. Let's change it back to non-search engine friendly URLs.
See a problem with the above? When you make those changes it's like moving your site again. Your URLs are totally changed, so that's like starting all over again. I imagine, if you switch, and then switch back and keep fiddling, you'd probably get penalized as your URLs are changing very frequently, and this affects all of your URLs to the forum as well.
