> Don't many users already have PayPal accounts, though?

Nope.

I have 3 options: PayPal, Google Checkout, and Credit Card (Stripe, but my users don't know about Stripe).

Approximate proportion in new subscription payments is: 15% PayPal, 10% Google Checkout, 75% credit card.

Many existing users are switching from PayPal and Google Checkout to Credit Card (because now I have Stripe option).