The proper convention is to include the possessive apostrophe even when the word ends in an "s." So "James's" is correct. The only exception to that are proper nouns so well established that traditionally they have always been used with just an apostrophe. The only examples that really come to mind of that are "Moses'" and "Jesus'". Otherwise, you always want to include the additional "s." Generally speaking, using only an apostrophe to show possession at the end of a word that ends in "s" is almost always wrong.