Audit 'sa' login in SQL Server 2005
I would say consider logon triggers to accomplish this. This way you aren't using a server side trace. You can also audit all sucessful logins in the SQL Server error log (right click on instance, properties, security and then choose both successful and failed logons.. I don't believe this shows you the DB context info, though)

Also - I would seriously consider upgrading to a later version of SQL if possible. SQL 2005 is two (three if you count 2008 R2) versions back. I know you probably know that but I'd feel bad if I didn't call it out :-)

I would also highly discourage the use of SA for any logons. SA is a highly privileged account. It is -the- highly privileged account in SQL Server. Everyone knows there is an account named SA and it can be prone to hack attempts. I tend to push for windows authentication only and ensure a group is added that the proper DBAs team can properly and securely added to. I often disable the SA account in mixed mode and will create another account with SA rights but a non descript name. If, for some reason, the SA account needs to stay around, I try and give that a horrible password and store it someplace really safe and not use it.

By not having an SA account used by so many people that you need to audit its usage, you can give more granular permissions to do the required activities in SQL and no more. Least privilege and tight access lists will take you much further than watching SA account activity. 

At any rate, Logon triggers may be the best bet. Trace would work but there is a cost associated with that (there is with logon triggers also but my guess is the cost of trace will be more expensive for you)