Replacement Graphics Card: Get 1 or 2?

I'm using Maya 2012 on a 2008 Mac Pro with two ATI Radeon HD 2600 XT cards in two slots. The set up is 2 Mac monitors and a Wacom Cintique connected to both video cards. I 'think' the Wacom is using one card and the monitors share another.

Question: Do I need to buy two of the same card or can I use a new one for my displays and keep the old one for my Cintique?

If anyone's curious, based on someone's advice online, I am looking at getting the Quadro VCQ 4000 Pro ($780) or the ATI Radeon HD 5870 ($423). I can't use Maya's HQ nor Viewport 2.0 without it crashing with a graphics card error, so I must get a new card. I'm just hoping I don't need two at those prices.