It depends on the database, and what it is used for.
If it's the back end for small website, or small business general orders/works database, then it really doesn't matter. Careful database design, use of appropriate indexes, use of stored procedures wherever possible, use of denormalization where appropriate, careful writing of SQL queries (e.g. DISTINCT queries may be very slow, and use of subqueries may provide the same result more quickly), etc. can all boost database performance greatly.
Unless you are working with millions of rows, or thousands of database updates per minute, pretty much any decent PC with plenty of RAM will do the job perfectly adequately.
For very big, or very busy databases, the optimal hardware may need to be determined by performance profiling.
