how slow a hard drive would be on the same channel as an optical drive.
That's what same channel means. A hard drive will only be slowed down by the optical drive when the optical is in use, otherwise it's unaffected.The only time you would actually see a difference is when you put 2 IDE's on the same ribbon in RAID
what if you wanted to play a game that was installed on that hard drive, with the game cd in that cd drive?