Oracle Database Upgrade Path

Oracle Database Upgrade Path:
The below document in metalink will recommend what are all required patches for upgrade.
Doc ID: 756671.1 : Oracle Recommended Patches -- Oracle Database

The below document is a nice document for database upgrade to 10GR2.
Doc ID: 316889.1 : Complete Checklist for Manual Upgrades to 10gR2

This document will give you all the details regarding database upgrade to any version.
Doc ID: 730365.1 : Oracle Database Upgrade Path Reference List

For the above documents you need to have a account with https://metalink.oracle.com/. The account with Oracle for this site is possible only when you have a valid Customer Service Identification (CSI) number available with you.