Here is a good article for upgrading to 1.6 version:
My advice: make a brand new installation of Magento 1.6.1 and then move your products and stuff from your current database on Magento 1.5.1. To help you out in moving your database, you can use this: http://www.shopping-cart-migration.com/supported-carts/55-magento
Before upgarding I upgraded all my widgets in Magento.
I updated with this: http://www.magentocommerce.com/wiki/1_-_installation_and_configuration/magento_1.5_to_1.6_upgrade and I got the error wich is mentioned in this forum several times (mysql 1052 and errno: 152 if I remember right).
After reading this: http://www.expertmagentodevelopers.com/blog/how-to-upgrade-from-magento-1-5-1-to-magento-1-6-0/
I also made the change in /app/etc/config.xml:
<initStatements>SET NAMES utf8</initStatements>
<initStatements>SET NAMES utf8; SET FOREIGN_KEY_CHECKS=0;
I changed the mage-file permissions to 775. After that I executed this command via ssh:
./mage mage-setup .
with the result:
(and something what already was in there and was not success because of that...)
Hope this helps: I´m not sure wich one of those the on who helped me? Mayby the mage-file permissions? and the xml? and the mage-setup? :D
And my Magento started working with multiple widgets in it.