Tempdb will use the collation of the model database when the server is started (i.e. tempdb is built).
Use can coerce the collations used in queries to compare using a specific collation.
= (colname collate collationname)
You can change the collation of a database by
alter database collate collationname
this will change the default collation for new tables
For existing ones I think you will have to run alter table scripts.
These may all change the representation of the data though.