Mac Pro Quad Core (2009) - Video Card Setup

I have purchased a used Mac Pro and want to setup a system primarily focused on PS5. I also want to venture into some AVCHD editing from my DSLR. For video editing, I will be using Premiere and perhaps After Effects. As mentioned, I have the Quad Core 2.66, I have bumped it up to 24 MB ram and I will be running dual ASUS PA246Q monitors. The computer still has installed the factory video card (NVIDIA GeForce GT 120 with 512 MB of GDDR3). I have ordered a PNY Quadro 4000 which leads me to my questions. I am looking for suggestions as I have no experience with configuring a system like this. Should I just replace the existing card or can I run both cards and see an added advantage? I was thinking running the monitors from the factory card and keeping the new card for processing. Any thoughts? Is there a know link to another post that covers this already? Thanks in advance.