Sounds like what you are looking for is something like the Common Public Attribution License which is known as a Badgeware license. It did get OSI approved but it's not going to be particularly popular. The only issue is the copyright notice displayed on startup vs bottom of a page and how it applies to websites (should be ok as it's listed as being applied on the graphical user interface), the worst that happens is someone goes to lengths to have the copyright notice disappear after the first page visit. There is also the Adaptive Public License.
The original 4-clause BSD license has an clause requiring advertising.
Most commercial opensource products normally work by dual licensing under a GPL/commercial license. They keep a 'pro' version that has more features, better performance and so on but that doesn't seem like it's feasible for your situation.
It's worth thinking about if you wan't to force the code to be shared (like GPL) or a less restrictive MIT style license with your added copyright notice requirement. For webservices provided over a network there is the AGPL license. Since the normal GPL only requires source to be provided when there is binary code distributed, the AGPL requires it to be accessible by anyone interacting with the software).
It's worth looking at the following:
Here is the Open Source Initiative's "Open Source Definition".
And there's the GNU foundation have a Free Software Definition
Also you shouldn't be writing a legal license unless your are a lawyer, or at least consult with one. At the very least you should ask in the the OSI mailing lists.
EDIT: One more thing is it's probably better to have an advertising badge rather than a copyright notice. An advertising badge makes it obvious that you make the forum software and gives people somewhere to go if they are interested in the forum software itself. It shows your not somehow responsible for the website. As such can drive more people to your company. Also it's possible that the copyright makes a claim of owning all the copyright on the site itself rather than just the forum code, more extremely it might even be that making such a claim is somehow legally impossible and invalidates your entire license but I'm not a lawyer.