I use Delphi 6 for creating simple SCADA systems for S7300. Im using PRODAVE from SIEMENS. From Prodave You need to import one dll library in which all functions for connecting and exchaning data with PLC are stored. It is also possible to connect with S7200 PLC.
It is also possible to use OPC server (software which allows communication with i.e. S7200 PLC). Siemens has such software: "S7-200 PC Access". I did not used this software but I think that it is possible to "mix" it with Delphi.