Your CPU is extremely old and is holding back your GPU.
The CPU can't process all the information the GPU is giving to it.
I would not recommend upgrading the CPU, the architecture your motherboard is using is far to old to be a decent modern day gaming machine.
I know it sucks, but your best bet is to buy a whole new system, you could use your GPU to help save on the cost's of building a new machine.
For starters your system ram is a little low and your cpu is an antique , you have a Pentium 4 ? With those components you are at the minimum requirements to run the games and when you are at that level , you can technicaly run the game but it's not going to be very good.
Call of Duty: Black Ops Minimum System Requirements: OS: Windows Vista / XP / 7 Processor: Intel Core 2 Duo E6600 or AMD Phenom X3 8750 or better Memory: 2GB HDD:12GB** Video: Shader 3.0 or better 256MB NVIDIA GeForce 8600GT / ATI Radeon X1950Pro or better Sound: DirectX 9.0c-compatible DirectX: 9.0c
As you can see your cpu is causeing the game not to play very well and you are lucky to be able to play it at all since it requires a core 2 duo as the minimum cpu requirement.