How many databases/codebases?

hi,
i have question sql server 2005.

suppose have hospitals application where patients records , other records stored. solution hosting one. here are three options:

1. we have 1 single database institutions (hospitals) need keep id every institution say  institution id. if need rollback transactions 1 institution how can while not rolling others? also, every institution may have different business rules i.e. 80% of rules , data is same every institution 20% business rules , data may specific institution e.g. if institution located in canada instead of us. meant say, every instituion has data not in base database we need add this data institution base database (i.e. 80% common part) best way / recommendataion resolve these issues? example taxes, i.e. tax system in canada, , eu different. can handle in same code base?
2. the 2nd option have separate database every institution. this will give rise maintenace issues e.g. if bug fixed 1 customer same bugfix has fixed every customer, 4000 customers. difficult maintain code base.
3. the third option base database same every institution , in addition base database there 1 additional database every institution has 20% rules/data vary. option may run problems if rule/data implemented in base database has rules type of transactions, there conflict , need manage feature list every customer , first see if execute in base database, if not use institution specific database. again raise maintenence issues.

 
what industry practice or recommendation and why? there material on web or on microsoft web site that gives guidance on these / similar issues. 
 
your cooperation appreciated.
thanks,
paraclete   

start here:
http://msdn2.microsoft.com/en-us/library/ms151198.aspx
http://www.databaseanswers.org/data_models/index.htm

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