SAP BW connect to legacy datasource

Hi,

our BW system is currently connect to SAP ECC 5. (our BW is 3.5). We have been asked by the user to connect to legacy system (Oracle database & SQL database) apart from ECC datasource. My concern is system performance & database sizing. What factor/information should I seek from the legacy owner inorder to evaluate the impact from this request.

Regards,

Rujira