PayPal accounts explained
PayPal accounts are either 'Personal' or 'Business'.
If you have a PayPal Personal account, then your customers will have to also have a PayPal account to be able to pay. PayPal Personal accounts are free to setup and use.
If you have a PayPal Business account, then you can accept payments by credit card without your customers having to have a PayPal account. The Business accounts are either Website Payments Standard (free) or Website Payments Pro (you have to pay a subscription). Either will work.
For your customers to pay without signing up to PayPal, they
must click on the (very small) link on the PayPal payment page just
above the card logos that says 'Continue'. It looks like this:

Additional info about Website Payments Pro accounts:
With a Pro account it would be possible to take card payments
directly within your bookwhen pages instead of being redirected to
the PayPal site, but this feature is not yet supported by
bookwhen.
Payments via Google Checkout and Amazon Payments are due to be implemented sometime in spring 2010.