When two Biblical passages appear to conflict, you have three options:
1. Adjust your interpretation of one or both passages.
2. Reject the Bible as hopelessly muddled and contradictory.
3. Reject one or both of the Biblical books that contain the offending passages.
My advice is to always err on the side of caution and go with option 1, giving God the benefit of the doubt by interpreting the passages in a manner that makes Him both truthful and faithful.
This especially applies to any passages related to God's covenants and promises.
Update: For some Common Sense Bible Study, check out http://jaycarper.com/biblestudy.