On the paypal site it says:
Our Logo can Make You Money.
Online Shoppers feel safer seeing the paypal logo.
Add it to your site today.
Do you think the logo can actually increase sales?
yes. Paypal is one of the most recognised payment processors on the internet. Add it with some credit card options, and those old whipersnappers might just fork over their details. That being said, the only way you will know is if you test. Chuck it in OpenX and see if it makes a difference