If your getting interference on your TV when your PC is on, that means your PSU unit is over-working itself. This is denoted by lines moving across the TV when the PC is on. Also, bad PSU shielding or no PFC (for the really cheap ones)
I have an Enermax Galaxy 850watt PSU. I don't get interference on my TV even after hours of play on my crappy CrossfireX configuration of two Radeon HD 4850s (yeah don't waste your money those, they suck.....get 5890s instead)