I will probably try PostGreSQL for my needs as it seem powerful enough
and easy to ship to my potential customers.
For the data provider i will try npgsql as you and others suggested,
as it seem enough reliable and stable to be used efficiently.
I will probably use newsgroups again later if and when i will have
more specific issue, in the meanwhile i go back to links you have
provided me to read faqs for pgsql and npgsql.
Thanks everybody for your help and suggestions.