should I upgrade the graphics card?

Hi guys. :thumbup1:
I am new here and this is my first post.
I have a intel pentium 4 processor 3GHz, ASUS P5KPL-AM/PS MB, 4GB DDR2 RAM, 500GB HDD and NVIDIA GeForce 6600 256MB graphics card, and 400W power supply. I am using windows XP SP3.
I was able to play Fable 3 with it but when I tried to play the GTA 4, the game play was very sluggish. The same thing happened with mafia 2 as well.
So will upgrading the graphics card let me play those games?
Also which graphics card will work best with my PC?
Any help / advice is welcome.
thank you.

Hey you can check out the graphics card that has been mentioned in the below thread, it seems to be similar case:
Suggestion on best and cheap graphics card for gta iv

Thank you. I am still trying to asses if I should upgrade the G card or the processor. My MB is compatible with both core2duo and core2quad. So am trying to decide if I should just upgrade the processor or the G card.

I think that should better change the graphic card as you are having a good processor.