MacPro GUI card for Resolve

I have a MacPro 4.1 with a GTX-570 processor card. I want to fit a single width card for the GUI - can somebody suggest a modern card that will work? I have been given a Quadro 600 card but that appears not to be supported. I need to support the full 2560x1440 screen size. It cannot need any extra power since I am using them both for the GTX-570.
I am on OS X 10.8.3

I've now been given a GeForce 210 card. This works but is very slow. My 4K to HD 25fps footage is playing at about 10 fps even though the GPU light is green. There is some discussion about this on the Internet with various hacks - I guess I need to do some reading! Any links to sites relevant to 10.8.3 much appreciated.

The best single width card I can find is the GeForce GT640. Is anybody out there using this as a GUI card in a MacPro? I cannot find a max screen size but assume it would suffice?

Don't bother. The performance gains of adding another card in your setup are so minimal, you probably wouldn't even notice.

Maybe I am doing something wrong? Here is the sequence:
1) Installed a single GTX-570 (Mac enabled) and got just acceptable results of playing a 4K file in real time with no nodes, but on a rather small computer screen.
2) Changed my GUI monitor for bigger Dell U2713H to support recommended full Resolve res. This pushed my frame rate down considerably below real time (i.e. GPU light was now red) using Display Port.
3) Added GeForce 210 card for GUI. The GPU light was now green but the play frame rate was now only 5fps also using Display Port
4) Just tried a GeForce GT520 for the GUI and my play rate is now back to real time. I cannot keep this card because it is two slots wide and I don't have the space.

Neither of these GUI cards are Mac enabled so I don't get a boot screen but they seem to otherwise work. Hence my request to get feedback on using a GT640 which I assume will be at least as good as the GT520. The other cards were owned by our IT dept but I need to buy the required card so I obviously want some reassurance it will work first.

I could be wrong, but your results are surprising to me, because I was in a similar situation recently. I had a single GTX570 (2.5GB), and adding a Radeon 7750 for GUI only gained me about 1fps. I spoke with Dwaine, who informed me that result made sense.

Do you have optimized display mode turned on? It should always be on for single GPU systems.
For 4K work, it's important to have a lot of GPU memory. So if you have a lesser version of the GTX 570, I'd suggest replacing it with the 2.5GB model to start with.

If you still want a second card, you might check out the Radeon 7750 as well. Single slot, no aux power. I'm not sure how it compares with the GT640 though.

OK - I have now installed a GT 640. I am slightly disappointed that it doesn't show a boot screen but this is not important. It is definitely faster and I can now try and optimise the system.
Can you explain what you mean by 'optimized display mode'?

It's under master project settings. See page 81 of the manual.
And when working with 4K, I think you'll find it makes a big difference to have more than 2GB of VRAM.

The GTX-570 does have the 2.5GB RAM.
I've changed the setting as per pg 81 and it is now playing 4K files at full speed so I am a happy bunny!

If I get some time I'll take out the GT 640 board to see just how much difference it makes.