OP- you're in a tough situation (and one I'm familiar with... I'm pagan, DH follows a loose christian/mystic path, and my parents are "orthodox" roman catholics who are very concerned about the religious path their grandchildren are on). There are a couple different paths you can take here...
On the one hand you can send them a book or books on your personal beliefs. I wouldn't send books that say "religion is bad" (this just puts them on the defensive, makes them more concerned about your choices, and isn't likely to "win them over" anyway) but look for books that explain in a positive manner what you do, in fact, believe. You can offer this information not in a "if you want to be a good grandparent you will avoid discussing x or you will encourage y..." but as a "we want to share with you what we will be teaching our children/what we believe about X and Y".
After a lot of conflict this is the path DH and I finally took... we joined a Unitarian Univeralist group and provided my parents with "this is UU" information. We explained that we don't mind my parents discussing their beliefs with the girls once the girls are older but that the
core values of UU were the ones we'd be teaching (there are actually several aitheists in our community, though the majority of memebrs in this group are buddhist, humanist, or pagan with a smattering of christian and jewish members), and that we'd like to avoid confusing the girls with "one true way" beliefs until they're old enough to understand that every person has the right to their own belief and that no one belief is any more valid than any other.
Another option is simply ignoring your parents/extended family's religious leanings and going with the "pass the bean dip" approach if they try to push their beliefs onto your family. So conversations about religion can be ended with repetition of the "this is our belief, pass the bean dip" or "this isn't something we're open to discussing, pass the bean dip" mantra.
We've used this approach too, and I've learned to open all gifts and cards from my parents since they often contain items that are not appropriate to our family or how we're raising our children. It's hard, especially with grandparents who feel strongly that they are saving their grandchildren and who don't respect your authority as the parent, but if you repeat the mantra early/often it can be effective. At least it can put the brakes on the more obvious "pushing" without giving the other party a place to argue from. You just refuse to engage and move on.
Of course, if you offer positive "this is what we believe, what we are teaching our children" and authoritative "this is not open to discussion, pass the bean dip" and still find that your parents or in-laws just can't stop... well, open letters, gifts, cards, and don't leave your children alone with their grandparents until they are old enough to discuss these subjects.
