Magento Stock Availability Message

We have a Magento webstore with our own theme. At the moment, we allow customers to purchase items that have a stock level of zero (we keep Magento stock levels up to date) but have a message that pops up in the shopping cart when an item has a quantity of less than 1 just saying that there will be a delay to dispatch. The problem is that on the item detail page, it says 'in stock' for all items, and there's no easy way for us or any customer to scan through and see all genuinely in stock items. We need: - the 'Availability: In Stock' message to change so it says 'Availability: 1-2 days' for items with a quantity > 0 and 'Availability: 3-5 days' for items with zero quantity - A new line saying '1-2 day dispatch' or '3-5 day dispatch' below the 'add to cart' button when browsing through a list of different items