The general answer is that it's better to put the OS on a separate disk since that means you can do reads & writes with the games/media disk and the OS simultaneously. This means that one doesn't have to wait for the other.
In practical terms, unless you're doing a lot of stuff at the same time, this will mostly affect load times and is less likely to affect the performance of what you're doing once the programs are running.
But if the programs you use are disk-intensive (batch photo editing, video editing, etc.) then more disks will help even after the programs have loaded. In that case it's definitely a case of the more disks the merrier.
If I was you, I would use the separate hard drive way like suggested above.
I would even further consider using eSATA and having the 1TB in an enclosure. IT is really convenient that way. The benefit of having your data, games, media etc. on another large disk is that if you somehow corrupt your OS or have an OS problem, a re-install won't be that big a deal. You won't be worried about wiping out your data/media etc. It's for peace of mind besides convenience.