Importing Data (Overwrite Any Existing Records)

Good Morning,  I need some assistance with SQL Server 2000 Importing Data. 

When I import data from a text on a routine basis, three things must happen:

1.  New records identified by primary key get appended to table.

2.  Exisiting records identified by primary key get overwritten with new/(updated) data.

3.  All other existing records are left alone.

Does anyone know how to Import Records with the following the criteria above?  It cannot insert duplicate primary keys by nature, so it must overwrite those records!

This is being built into a DTS Package, but I need to get over this obsticle!  Thanks for any guidance!