postgresql upgrade/downgrade -- help!

[will trillich <will@serensoft.com>]

<moron>
i munged my sources.list to point to fundet.no for the latest
potato-friendly postgresql DEB package, and upgraded via apt-get
update && apt-get upgrade.
</moron>

<moron level=2>
it broke my databases, so i tried apt-get --purge remove
postgresql<various7.0.3potato*> while hanging on to the
/var/lib/postgres/data area. then i used dpkg -i *p*sql*6.5.3*deb
to try to recover, and there's more broken stuff than ever.
</moron>

<idiot>no, i don't have a recent-enough backup</idiot>.

is there a five-hour intensive plan that might restore my
postgresql 6.5.3->7.0.3->6.5.3 database? suggestions welcome
(i'm crafting a database-backup plan now, in the meantime.)