YOu can configure different types of NAT on ASA5520,
As you mentioned that you have a public ip address rage alloted for the internal uses you can bypass nat all together as well.
Please refer to the following link for more information on configuring NAT, it is must to understand what we are doing first
http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/nat_overview.html
A quick suggestin to use PAT.
Since your inside users wants to go to outside network I believe via OUTSIDE Interface only!!!
you can use the following commands
nat (inside) 1 0.0.0.0 0.0.0.0
global (outside) 1 interface
1 is the NAT-ID here
it can be different as well, change it to any number if already used.
Please post if there are other specific questions.
Sachin