You'll need a current license to upgrade the signatures (but not the OS, which comes with signatures).
The minimum required version for upgrading to 6.0 is 5.1. The minimum required version for upgrading to 5.1 is 5.0 The upgrades from Cisco 5.1 to 6.0 and Cisco 5.0 to 5.1 are available as a download from Cisco.com.
http://www.cisco.com/en/US/docs/security/ips/6.0/installation/guide/hwObtSW.html#wp1032104
To see events you should grab a free copy of the IME to watch your events:
http://www.cisco.com/cgi-bin/tablebuild.pl/ips-ime