Heh, an 8600 is much more powerful than an 8400. To be honest, its roughly twice as powerful if I'm not mistaken.
Hm...after the war...that means more explosions etc from all the soldier/demo spam. Thus, I say it's your CPU since the Source Engine isn't as GPU-heavy as other game engines out there.
Here's what I recommend. Raise your graphic settings up a notch and see if your FPS drops. If it doesn't, then it's primarily a CPU bottleneck. Use the console (check the box in the settings, under "Advanced" I think, to enable it). Then type "showfps_1" (without the parenthesis) to get the FPS to show. See how that goes.
If what I said is the case, then since your CPU is Socket 775, which is technically out of date, you could buy yourself a faster processor. Also, since the prices on Core 2s dropped ever since Nehalem-based CPUs (i7, i5, i3) were released, you could definitely buy a really fast Core 2 Duo,
such as this one. Or, if you'd like, you could always go Core 2 Quad and future proof yourself with 2 more cores (but with slower speed).
Let us know what's up.
Fine Print: Upgrading your CPU will boost FPS but I do not guarantee it will boost eye candy.