Right now I have an EVGA NVIDIA GTX 680 (4GB) card hooked up to my 30" monitor running at 2560x1600 via DVI for FSX. I have a 37" HDTV sitting around collecting dust and was wondering if it's possible to hook the TV (HDMI) up to my same graphics card so that I can use it independently as a 2nd screen to place up various gauges like FMC/PFD/MFD, or use it for other apps such as moving map or flight planning? If so, will this make slow down the performance of FSX or no?

Hook it up to the HDMI port.

 

You might see a drop if you move gauges or views to it.

What if I use the TV solely for apps such as FSCommander for flight planning? WIll it hurt my FSX performance?

There's really only one way to find out...why not actually be bold and try it and see for yourself?

I use a 40" HDTV which I added recently and keep my 23" monitor as well.  I run, PlanG, FSC, GSP etc or move them over to the other monitor.  I have had really no impact on fps that I can see.  I find that other programs running in the background are the biggest impact on my sim performance.  I get best flying results with a cold reboot and going right in to FSX.

 

I also have AlacrityPC to shut down programs that might impact my FSX performance.