Ken,

Encryption is a component of Ipsec framework, your question is very broad which can cover many componets and under different kinds of circumstances such as RA vpn, l2l vpn etc.., however, I will try to help in providing a link and some basic examples.

The level of encryption depends on the type of encryption algorythm is used in the transform set configuration given on any device or firewall supporting Ipsec standard. A transform set could have AES algorythm providing 128 or 256 bits CBC, or DES 56bit encryption algo, or 3DES 168bit, look at the transform set if any configured in your firewall.

Reference this link for understanding Ipsec and encryption basics and important terminology,knowing these you can identify how strong encryption is implemented across any platform that support Ipsec standard and is configured with as such, not just asa5505.

http://www.cisco.com/en/US/tech/tk583/tk372/technologies_tech_note09186a0080094203.shtml

HTH

Rgds

Jorge