dual head or dual card

Question for Wizzards:
My main computer (FS+MCP) drives the outside display from a dual head video card.(1 head only used)
I want to add a second 8" monitor to undock the ATC text.
Is it better, from a framerate point, to use the second head of the card or add a second video card?

I would use the other output on the one card. You shouldn't get any frame rate hits for the ATC text. I've got 3 video cards with 5 outputs and my frame rate is in the mid 30's and I've got it locked at 35. This with Wilco/Feelthere's ERJ-145.
Regards,
Brian