There are two steps to deciding if you should be taking aspirin for prevention of heart attack (do not follow any of this advice without talking with your physician first).  The following guidelines are from the New England Journal of Medicine, May 9, 2002 (volume 346, No. 19).  First determine your 10 year risk of having a heart attack.  To do this, fill out the simple form at the following link:  10 year risk of heart attack.  

The dose should be a baby aspirin (less than 100mg per day).