If you look at what top etailers are using as text on the action buttons (thanks to ) you will find it obvious that the right call to action to be used on this type of buttons it should be Add to Cart.
Button Text Button Graphics
Add to Cart 58.0% None 48.2%
Add to Bag 9.8% Arrows 17.9%
Add to Shopping Bag 9.8% Cart 14.3%
Add to Basket 6.3% Shopping Bag 7.1%
Add to Shopping Cart 4.5% Plus Sign 5.4%
Buy 2.7% Combo 4.5%
Buy Now 1.8% Unique 1.8%
Add Item(s) to Cart 1.8%
Add Item(s) to Bag 0.9%
Add to My Bag 0.9%
Add to My Brown Bag 0.9%
Add to My Shopping Cart 0.9%
Order Now 0.9%
Source: Get Elastic-The Ecommerce Blog

But, like any other elements of your website (and especially with call to actions), it will be better if this button can look/display different texts based on your business rules and customers? expectations. It should persuade them to take different actions, in different places based on your business objectives.

Let's analyze a little bit more in detail this two call to actions:
The ?Add to Cart? button:
- suggests that the buying process that does not starts by clicking on the button
- It is not very imperative
- It creates the subliminal idea of a cart (you don't use a shopping cart to buy one product)

The ?Buy Now?
- It's very strong and imperative
- It suggests that once you click on it you will start the checkout process
- It suggests somehow an emergency

My suggestion is to display both of them, one at the time, depending on your business objectives:

1. If you wish to increase your average items per order, use the ?Add to Cart? button.
2. Analyze your average order value, also. Does the visitor meet your desired value? If so, make him start the checkout process by switching to a Buy Now button.
3. If you have a reliable data about your average order size and for years that number was 1.1 items/order you should use Buy Now and not Add to Cart.