Secondary graphics card

Hi,I just had a thought today and I wanted a second opinion from you fine chaps.I have 2 screens, a 19" secondary monitor and a 27" primary monitor. Would it cost my performance in fsx to plug in a second graphics card (a really old thing I've got lying about) to drive the secondary screen, and use my 470 (soon to be a 580) to solely drive the primary monitor?The old card doesn't require external power and is just a cheap thing I have left from my old system.