AC3 is being choked by your GPU.
Your CPU and modern games: A guide to those building.
My 6000+ paired with a 5850 plays BF3 at 59FPS solid and it's more demanding than AC3. Currently my fiance is playing Dark Souls 2 flawlessly at 1080p with the same 6000+ and 5850.
Games are pretty much 99% GPU dependent and single threaded apps will be slower on the Phenom due to its IPC being much lower and also its clocks are slower than the 6000+. To simplify that, the Phenom 1 quad will be much much slower at gaming than your current 6000+. A more realistic upgrade would be a better GPU as the 9800GTX+ is a little long in the tooth these days. If you absolutely want to retain your current setup then a Phenom 2 555 or 955BE would be a much better choice. Even still, at 1080p your 9800GTX+ will still choke the game up on higher settings.
"Resolution is just a number." #Ubisoft Origin/Steam
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