VACUUM and read-mostly tables

I have a near-real-time system writing into a Postgres 7.4.2 database
on the order of 340 million rows per day in about 300 million
transactions.  So we quickly bump up against the XID wrap-around
issue.  To address this, we divide the tables into 24-hour periods.
Once we roll over to a new period, the old tables are read-mostly.
We then run vacuum every 24 hours.

The problem is that we are writing rows every 1/15 second, 24x7.  There
is no down time.  I'm wondering if there is any way to avoid vacuuming
the old tables over and over.  The documentation seems to indicate that
a full vacuum is needed to avoid XID wrap-around.  Can vacuum freeze
help me?

Thanks,

    --Ian