To install Cognos Data Manager below are minimum hardware and software requirement. Refer this article to know about how to install Cognos Data Manager.
RAM: Minimum 1 GB
Disk Space: Minimum 1 GB free space.
Microsoft OS (Cognos Data Manager Designer works only in windows environment)
Cognos Data Manager 8.4
Database. For this tutorial series we are using SQL Server Database. Check this article for supporting OS.
If you are planning for a practice environment we are going to recommend please use a system with minimum 4 GB Ram.