paertially restoring a DB

i am having a problem with dump and restore. basically i have 2
databases

maindb and tempdb

the databases are almost the same but the main db has some extra
tables which dont change. what i need to do is get all the information
form the temp db and put it in the main db. the tempdb should
overwrite all the data in the main db, but the tables that exist in
the maindb only should remain

ie
maindb has tables:  dbsettings, data1, data2
tempdb has tables:  data1, data2

then the tables (and data) data1,2 from tempdb should overwrite their
corresponding tables but dbsettings should remain unchanged in maindb.

any things ive tried overwrite all the tables and i loose dbsettings
table completely (not good)

any help appreciated

Niall