No contest: postgreSQL under un*x.
MySQL4 is coming, but they have a way of making sure to never include the most usefull functionality.
Accounting cannot afford to have inconsistant data, which means your database model must ensure consistency. MYSQL simple is not capable of doing any kind of integrety checks, so that rules out MySQL.
Access is never an option. (a toy that is only good for learning the very basics of databases)
That leaves postgreSQL. PostgreSQL does have a full set of referential constraint features, plus stored procedures, subqueries etc.
But most importantly it has transactions, foreign-keys and constraints. In a good database design, these three will ensure that you have to f*ck up massively to get inconsistant data into the database.
Foreign keys ensure that you never refer to non-existing data, constraints ensure that you never enter data that does not match the requirements (think of out-of-range values, dates in the wrong format, zero-length names) and transactions ensure that if you need to do several queries in sequence, you will either do them all or do none. (lowering the price for 5 different products by 5 different amounts takes 5 queries, but if one query fails, the other changes are 'undone')
