Are you using x86-64bit
Thanks Baris for your fast reply
This for Oracle level instillation , but i need some advice in OS level instillation about spaces of HD and RAM
firstly you can use this document to install grid infra.
Also you will find information about hardware requirements on it.
You can get all the details at
RAC and Oracle Clusterware Best Practices and Starter Kit (Linux) (Doc ID 811306.1)
It has link for 11gR2 RAC installation guide as well which explains installation steps in detail