You should use:
$UpdateQuery->bindColumn("FeaturesBox", "s", "".((isset($_POST["FeaturesBox"]))?implode(", ", $_POST["FeaturesBox"]):"") ."", "WA_BLANK");
Then when you display the checkboxes you can use:
<input type="checkbox" name="FeaturesBox[]" id="FeaturesBox__1" value="1" class="formCheckboxField_Standard" <?php if (array_search("1", ((isset($_GET["invalid"]) && ValidatedField("Productupdate2","FeaturesBox"))?ValidatedField("Productupdate2","FeaturesBox"):explode(", ",$UpdateRS->getColumnVal("FeaturesBox")))) !== false) {echo "checked=\"checked\"";} ?> tabindex="1" title="Please enter a value.">