if you are running at 1280x720, you will likely only need one 4870 to hit 60fps on a Tv. Its like putting a 50hp motor on your lawnmower. Sure it has tons of gpu power but it only takes 5hp to cut the grass:)
I think Disco Duck's point stands - you really shouldn't waste money on more than one 4870 for 720 res. It's pointless to do more.