HELP database files are incompatible with server

Hi
I am posting again
I did a very small but terrible mistake with my database.
I installed postgresql and DSpace for use in the INSTITUTO ANTONIO
CARLOS JOBIM, dedicated to the work of this famous Brasilian composer.
I first installed a version 8.0beta5 of postgres but a software advisor
asked me to use a formal release and I installed version 7.4 and begin
to work with DSpace. After having submited about 200 items to DSpace a
researcher at the institute and myself found a bug when brousing the
archives. the log complained of a variable XactIsoLevel. and I learned
now it may be related to the JDBC driver.
Now comes the big mistake. I tryed to start the 8beta version of
postgres in a folder named /usr/local/pgsql2. it did not workand I
noticed postgres was trying to access the folder /usr/local/pgsqlwith
the 7.4version installed.after that I renamed pgsql as pgsqlbk and
renamed pgsql2 as pgsql and started the new version without a problem.
I noticed DSpace had still the same problem when brousing. (note that
this version of the database is empty) then I renamed the folders for
the original names, with pgsql containing the 7.4 database with all the
records for DSpace. but it never started again. it gives me the
following error:

FATAL: database files are incompatible with server
DETAIL: The database cluster was initialized with PG_CONTROL_VERSION
74, but the server was compiled with PG_CONTROL_VERSION 72.
HINT: It looks like you need to initdb.

is there a way to recover the database?
thank you for any help
Paulo Jobim