EXTERNAL GRAPHIC CARD/RACK

Hello,
MY NAME IS VITEK

I`VE GOT HP PRESARIO A935EA WITH 2GIGS RAM,PROCESSOR PENTIUM 1.8GHZ AND INTEL 965 EXPRESS CHIPSET256MB.
I`D LIKE TO ASK IF IS ANY EXTERNAL GRAPHIC CARD ON THE MARKET TO GET BETTER GAMING EXPERIENCE AND IF YES,HOW MUCH DOES IT COAST ?
THE BEST GAME I CAN PLAY THESE DAYS IS CALL OF DUTY 2 ON VERY HIGH DETAILS AND 4 WITH VERY LOW DETAILS.
PLEASE ANY ADVICE WILL BE VERY APPRECIATED.

THANKS