@Jonathan078NL We don’t know yet if the game is CPU or GPU bound. My son has my old Q6600 rig (non-overclocked) and it chews through all the Rome 2 Total War he can throw at it. I’d be surprised if you have a CPU issue, but it might depend on how much multi-threading the game supports as your networks get complex.
Not so sure about about your 9600GT though. The Steam page lists an 8800, but how playable a 512mb card will be once you have a bunch of assets loaded is another question.