I am not sure how familiar you are with Cisco firewall (ASA or PIX), however, i would recommend that you upgrade it to either 8.0.5 or 8.2.2, instead of the latest 8.3.1.
Reason being with version 8.3.1, there is a complete change of the NAT, so if you are familiar with the older NAT configuration, you will need to re-learn the new NAT feature in version 8.3.1.
Here is the release notes for version 8.3.1 for your reference:
http://www.cisco.com/en/US/docs/security/asa/asa83/release/notes/asarn83.html
Here is the configuration guide for NAT in 8.3.1:
http://www.cisco.com/en/US/docs/security/asa/asa83/configuration/guide/nat_overview.html#wp1122015
As version 8.3.1 is a new version, there tends to be more software bugs, so it might be better if you upgrade it to the earlier version of 8.x code (either 8.0.5 or 8.2.2).
Hope that answers your question.