Processor: AMD Phenom 9550 Quad Core 2.2 GHz
Mobo: ASUS M2N68 PLUS
RAM: Kingston 2x2 GB DDR 2 800 Mhz
GPU: ATI Radeon HD 6790
PSU: Thermaltake Litepower 500 W
Problem is that I am getting low FPS in games, regardless of graphic settings. For example, I started playing Prototype 2, and i get around 30 fps, but when I start butchering people it drops to about 20 fps or less. I installed MSI on screen display and it shows me that GPU usage never goes over 60%. All latest drivers are installed.
I think of changing my CPU to Phenom II 945 be or 965 be. because my mobo supprots AM3...
Would this be the right move or should I try something else before changing the CPU?
If your GPU usuage isn't going above 60%, that's the classic sign for a bottle neck. Now if your lucky your motherboard will support AM3 CPU's even if its AM2+ socket (mine is this way), then you won't have to buy a new board just a CPU.
Good news you can step up to an AM3 CPU with that motherboard! Make sure you have the newest Bios version installed, if not get the from the ASUS site and flash them. Make sure you know what your doing first because if you do something wrong, you'll kill your board and have to buy a new one anyway.
IMO updating the BIO's is the scariest part, I don't like to mess with them. One other thing, there are a few different AM3 CPU's that will not work with your board because they do not have the DDR2 memory controllers in them. I don't remember which ones they were exactly, but I'm sure you can find a list for your board of compatible CPU's.