It does sound kind of like a cpu bottleneck. I tried using a quad opteron (octocore 6128's @ 2.38 Ghz) to run l4d2 and found that my framerate poor. I was running a 7970 and it would only run at around half load. This could be what you are experiencing. May need to convert that dual socket rig to folding and get a single socket board for games.
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