I won't “re-invent the wheel” since so much has been written on the subject by others, but I'll summarize the “Mary had other children” question by saying that while in the word “brothers” (as in “brothers of the Lord” in Scripture) COULD have referred to any male relative of the Lord (i.e. cousin, step-brothers, etc.) So, it IS biblically possible that Jesus was an only child of Mary. Couple this biblical possibility with the unbroken Tradition of the early Church on this subject and you've got a pretty strong argument for Mary's perpetual virginity.
You can read more about it here: http://www.catholic.com/library/Mary_Ever_Virgin.asp.
Cool. I accidently posted a link to some early patristic writings on the matter, but the biblical facet is better handeled here: http://www.catholic.com/library/Brethren_of_the_Lord.asp.
Happy reading! ;-)