first of all your cpu will definitely holding back the card. secondly most of recent games out there need at least dual core processor to get decent performance. from my personal experience upgrading my 3.0ghz P4 to 2.93 core 2 duo almost double my frame rates when paired with the very same gpu (the gpu in use is nvidia 9600GSO which i believe have comparable performance to GT240).
I had a P4 3.2 GHz with a 9600GT, and so I upgraded to a Pentium D 945 (I had a Dell computer and the Pentium D was the furthest it would let me go) and I got at LEAST 50% more FPS in each of my games. It was pretty amazing the performance difference between the two.
Now I have a Quad Core system with an outdated GTX 460.