I am going to be launching the lastest version of Zencart soon and I am thinking about adding Paypal. I do not know anything about this form of payment so I am wondering if it is worth getting set up since I already accept 4 major credit cards.

Also, I do not know which one of these payment modules to Install.
Can anyone give me some advise?

PayPal Website Payments Standard - IPN
PayPal Website Payments Pro
PayPal Express Checkout