One thing to make sure of is you have enough RAM to fit the game and it is not going to your page file (disk). If you have 4GB you should..... be ok, but sometimes if you have alot going on in the back group it could cause weird studdering as the game is also loaded into your page file. Just something to check. If you have 6 or 8 GB you are probably ok.
However, typically it is due to your video card. I would suggest turning down your graphic settings to accommodate your GPU speed. Some games hit your system harder when they get into wide open scenes with a lot going or or dust, debris, shadows, etc. A GTX560Ti is still a very capable card.
There are also some games that do not run as well on the older CPU architectures. Not saying the Phenom II is old, but some games just run better on the newer CPU's and typically favor Intel. However, like I said there are only a few that do this and should not really vouch for needing to upgrade your CPU and consequently your motherboard as well. As mentioned before, a Video card upgrade is typically the area to upgrade for increased graphics settings and better frame rates. You can also always try your hand at overclocking your GPU. It should have some headroom to increase performance.