How can i speed up my computer?

Hi all; so I dunno much about computers and I rarely ever play games but i decided to buy arkham asylum because of all the good things I hear about it but its SO SLOW! It lags every few seconds. Do i need to change the graphics card? I dont know the exact model of the card as i dont have the box but i think its called ‘rampage go to hell’. it was a gift last birthday and so I don’t think its that good.