Hi & Welcome.
You should see a pretty good boost from games upgrading to a 7600GT.
What CPU speed are you running? Also how much RAM (expensive RDRAM) do you have?
You will need to upgrade your PSU (Power Supply Unit) from 250w to at least 350watts to make sure the 7600GT is reliable. If you need more RAM or a faster CPU also, it may be time to look at a full system upgrade. BF2 has fairly high requirements to run smoothly with a good level of graphics.
Edit: I type slow. haha
Edit 2: The 2.2 CPU should be OK, but BF2 really likes ram - lots of ram. One gigabyte is usually the minimum for the game to run good.