Graphics card question(s)

I do 3d animation on my computer and when I am rendering, I like to keep an eye on it. But when I do, I can't really do much else with my computer with out effecting render speed. I am considering adding a second graphics card to my computer so I can dual screen without effecting my performance. All I am looking for is to have my render up on my computer monitor and and have the tv (right next to the computer) play movies or go on the internet.

My question is, do I need another same stat gpu for the tv or can I have less expensive get-the-job-done graphics card (~$100) without hindering performance too much.

My current gpu is the EVGA GeForce GTX 680.

Thanks for any and all responses!