Recently I had to upgrade client wordpress installation from 3.0.5 to 3.4.1,which the current stable release.
The problem I faced is that I was always getting permission denied error : You donot have sufficient permission to upgrade.
This error comes even after I login as the admin.
Then after a long search in the google I found the following link:
So this is what I did based on the instructions.
1. Installed Phpmyadmin and accessed the wordpress database
2. locate wp_options table. Browse the data and look for db_version.
3. The database version written is 15447 . What you need to do is change that to 8204.
4. Save it . Restart apache
Then again try upgrade the wordpress. First it will ask for the database upgradation subsequently it will lead you to wordpress upgradation to the latest version.
Hah.! Saved lot of time and pain of manual up-gradation. Hope this help you the same way!.