Thanks to all for your suggestions.
I am planning to go with Seagate 1TB HDD(ofcourse with 7200rpm)
So i understand that installing internal 1TB HDD will not affect the booting or the system performance(speed).
At present i got 250GB HDD fully loaded, so i am upgrading to 1TB.
So to boot the system i will be using my existing HDD and 1TB will be my WAREHOUSE.
That wiil be for my project backups, pictures, music, movies, some software backups, etc
So external HDD will be connected most of the time to access high resolution pictures and video clips for my projects.
So what you suggest internal or external?