I apologize for resurrecting this old thread
I have a 2009 quad core that originally used a gt120. In August '10, I replaced it with the 5870 and had been living happily ever since. I was being an insomniac tonight and was looking through various upgrades (3.33 proc, ssds, etc.) and ran across this thread running both cards simultaneously. I decided to give it a shot and reinstalled the gt120 in slot 3 (with the 5870 in slot 1). Everything is working great with both installed either in mirrored or extended display.
That being said, I lose roughly 3 FPS when full-screen gaming by having the 2nd display plugged into the either card with extended desktop (game running on the 5870). I lose an additional 10 FPS or so when running in mirrored mode if the 2nd display is plugged into the GT120 but not when the 2nd display is plugged into the 5870 (it remains the same as when I run extended).
It isn't an issue for me personally, because I don't ever run mirrored - but I am interested as to why I take more of a penalty when using additional hardware.
Anyway, I thought I'd but this up here as another anecdotal confirmation of success, with the caveat that the performance was not quite as I'd expected.