SQL Server 2005 SP2 contains Reporting Services -MOSS integration feature. Reporting Services can be configured to run in Sharepoint Integration Modew where in reports can be managed from MOSS and not from Report Manager.
After your Reporting Service Report is deployed to SharePoint Report Library you could configure the report to use the common datasource from Data Connection library. The Data Connection library is used to store the datasource files that stores connection info to connect to RS application database (SSAS db / Custom SQL DB etc..) in a central and common location.
By default the .Rsds file content type does not appear as a new item dropdown. You will need to configure the Data Connection library to include the Report Data Source Content type to have it appear in the New item dropdown.