Pessimistic locking approach in our 4 years old web application has worked its time.

We definitely need to migrate to an optimitic approach.

While reading the few threads about MVCC, I realized most of them are 2 to 4 years old.
And none would give a clear explanation about what to consider when trying to choose between Hibernate versioning or DBMS native MVCC (or both together, why not?)

I'm wondering if arguments have change today:
- has PostgreSQL MVCC gone better in the past years?
- is Oracle MVCC still the most performant one?
- if MS SQL Server 2005 MVCC peforms poorly, what for 2008 version?
- how does that compare or work together with hibernate versioning?

Last but not least, when you suffer too many locks on your DB, would you consider migration to optimitic locking the first step to solve the matter?

lots of questions, I know, any opinion is welcome.
I'm sure the optimitic locking implementation problem is interesting for many of us.