The reason why asked about the version is that there some integration methods that may not be available in some versions like the Webservices that Rich mentioned is not available in 46C. Idocs is not an outdated technology, but it is more geared towards EDI interfaces rather than other interfaces.
You have to first differentiate between your conversion/bulk load requirements, online/real-time requirements and your batch interface requirements. Depending on which one you are talking about, the choice of the interfacing tools will differ. No one tool is good for all. RFCs/BAPIs are a good way to do real-time interfaces with users online, via business connector. But this is resource intensive, so you should not use it for bulk data transfers. You can still use the same BAPI or RFC as a simple function module call in a custom program and schedule the custom program to load data from the external systems through files during off-peak hours. This way your data is loaded into the system while it is not interrupting with the online users for resources.
IDOCs/RFC/BAPI/BI/DI/Call transactions/standard SAP program are the different interfacing/data conversion technologies you can use. Don't forget the powerful LSMW tool which is really easy to use and powerful enough to have complex mapping done and flexible enough to write some code to achieve some specific mapping transformations.
Bottom line is you have to evaluate what is best for your case and use it. It can be a mixture of all.
Srinivas
