I think that forums impact the hardcore demographic that some companies are targetting!
Example when Sega was looking for new game idea's they used polls on the Sega boards to descide which direction to go in. I took part in these polls. Example of questions were which Sonic Character would you like to see in their own game Shadow , Tails , Amy or knuckles the answer was Shadow then later onanother poll question came out would you be objected to Shadow with a gun and should a game based on shadow be a dark experiance. These different Sega polls I feel shaped Shadow:TheHedgeHog considering the polls were put out by Sega of America andso was the game. So obviously Sega feels they do.
Forums have the ability to affect about 200,000-500,000 buyers in my opinion. Why so little because out of the entire gaming industry how many consumers are on online forums talking about games, not that many only the die hard industry lovers. Fanboys/loyalists will always support their brand so forums don't really impact their choice in games. However Loyalists and intellectual gamers can have a profound effect on the casual gaming segment that may come onto a forum.
Another perfect example would be EarthBound was a sleeper hit in North America not that many gamers bought it but it was loved by any RPG fan at the time. Now a few years later (Decade) you go on any gaming forum and mention EarthBound and you have dozens of people talking about it. On Nintendo.com theirs at least a thousand or more die-hard EarthBound fans and thousands signed the latest Starman petition. More then half the people on N-Sider and other forums have never even played the game they claim the want to buy it so badly they've heard so much about it!
Just like EarthBounds growth in popularity I'm 100% sure that forums impact the hardcore demographic which intern can make or brake the sales of a game. However it cannot create a hit like Halo (4 million copies) or Halo2 those are word of mouth. Forums don't reach that many consumers only the true and die hard gaming audience!
-JC7
"In God We Trust - In Games We Play " - Joel Reimer