Basically, yes.
It would say, "UI card being used for processing" or something.
This means, inbetween doing color calculations, the card also has to redraw the UI, display the images and maybe even the scopes.
This will take up to 50% of your theoretical performance.
It sounds your main focus is actually CS5.
This means, you NEED to have the Quadro 4000 connected as your main display, forget the GT120 then, it has like 10% of the power.
But if you can live with the old card as your main card, the Quadro 4000 will do a proper job driving davinci and davinci ONLY.
A slice of color...
DaVinci 8.0.1b3 OSX 10.6.8
MacPro 5.1 2x2,4 24GB
RAID0 8TB eSata 6TB
GTX 285 / GT 120
Extreme 3D+ WAVE
http://www.saschahaber.com