If he takes frequent backups redundancy shouldn't matter. While he does double his chance of total data loss, with a loss of a single drive, a backup will prevent this. A gaming machine usually won't have anything mission critical on it (games), so it can probably spare some downtime when one of the drives fail.
What exactly is filling most of your disk space? Can it be downloaded again? Assuming it can.
You could set up 2x2tb drives in raid 0, and use the 1.5tb drive for automated backups of files that couldn't be re-downloaded in the event of a drive failure.