Clopidogrel and aspirin are commonly used in combination to prevent heart attacks and to prevent blockage of stents. Both clopidogrel and aspirin work by preventing platelets (sticky cells that circulate in the blood) from forming blood clots in the arteries supplying oxygen to the heart and in stents. The investigators hypothesize that aspirin 325mg compared with aspirin 81 mg will increase blood levels of the active metabolite of clopidogrel in patients with a history of coronary artery disease who receive a 600mg loading dose of clopidogrel.