The "icon next to post type" is definitely the way to go. So, for example, in your "Valve" forum there's a little icon next to the thread to indicate what its about; Half-Life, Portal, Team Fortress, etc. Same goes for "Other Gaming", a little icon to represent whether or not its PC, X360, PS3, DS, or PSP, and something to signify "general" or "multi platform".
Off Topic doesnt really need icons since everything goes.
A system where more threads exist in a single section of the forum should help increase post rates, and at least the illusion of an active community. The more you segment the forum the less likely people are to go into certain areas to post, thus lowering the likelihood of people posting in particular topics.
Here's a rough mockup of how our current news forum channel would look with an icon system.
Essentially you'd have all Half-Life, Portal, Team Fortress, Left 4 Dead, Steam, and Valve news collected in one forum, with the appropriate icon next to each thread. You'd have another channel for Other Gaming, with appropriate icons for systems, and a final channel for Off Topic discussion. Maybe one last channel for HLFallout specifics such as site news, bug reporting, organised games, etc.