Generally it's not a good idea to run two different manufacturers cards like that.
What negative consequences might I face? What would you recommend for someone on a mac pro who needs 4 monitors, and would like a quadro 4000 but can't afford 2.
Can I get the processing boost out of the 4000 and use it drive 2 monitors, and then use my ATI card to simply drive the other 2?
Yes, this works fine. My current Mac Pro was configured like this for a while until I overhauled its video. You won't run into any problems.
The comp seems to be running in this set up, though I am getting errors in After effects. It says my graphics card is the quadro4000 and AE runs, but when I go to Raytrace mode, I get an error, and I was getting OPEN GL failure errors when using Optical flares. I am unsure if this issue is stemming from the mix of cards or not...
Will investigae, but open to suggestions.
I know this thread is a little old, but did this setup end up successful for you? The ATI 5770 and Quadro 4000?
Were there any setup procedures or pointers you would give for getting it up and running smoothly? I've ordered the exact setup for my new machine. Do you have to use a power splitter to run both of the cards?