postgres 8.2.3 transaction xid wraparound

I have been using postgres 8.1 for a while now with large databases
(over 300-400G). The biggest issue I have faced is the routine vacuumdb
command that takes care of the transaction id wraparound problem (I run
this once a week on each postgres box and it takes about 24-48 hours
depending on the load and size of databse). (autovacuum just doesn't
work for me as I need to routinely shut down postgres briefly and
restart)

I run the following command to determine when to kick of vacuuming..

Select datname,age(datfrozenxid) from pg_database

With 8.2.3....what is the query I need to run to check age of xid for
wraparound purposes (I read from the docs that it is now maintained on a
per-table basis..does this mean if I were to vacuum tables
individually..i would not need to run a vacuumdb on the database
cluster?)

Thanks

Sriram