The CPU will hold that card back a bit, as it is a Willamette core, but you should be getting much better frame rates than what you are getting.
In the task manager, under the processes tab, how many are listed (look at the bottom left area of the dialog box). Now, count the number actually listed and see if there is a descrepancy.
Next, in the game itself, how much video ram does it show being used against what is available?
What is the AGP aperature size in the BIOS rom set to?
Let's go from there.