I've had two GT120's in my 2009 MacPro/12Gig 4x450GB15K RAID for over 18 months with no KPs...but mostly running one display...a 27" iMac using the Apple proprietary cable...the miniDP version of this cable:
Upgrading to the varios OSs along the way to 10.8.4.
No problems. On for months at a time.
Recently, I got an LG 27EA83-D that does 2560 x 1440 (same res as iMac) native running off the DVI of the GT120 as I deployed the iMac at my office and needed a panel for the MacPro. No problems.
Then I bought a second LG panel, discovered what many know here that you can't run 2560 x 1440 off the passive miniDB to DVI or HDMI...so I plugged it into the second GT120 DVI port. Rigged it actually for four panels...Powerpoint is totally wierded out...doesn't paint or update correctly if the second display is running off a different board. Hmmm...
So I get a KP while I'm "doing nothing." Hmmm... Then another. Then another. Then another...
I'm getting them consistently using Safari and LogMeIn. Can almost set my watch. Hmmmm...
Anyways, I too ordered the 5770...hoping to drive four displays...two at 2560x1440 and two at 1080p.
What else do I need?
Thanks you for everyone who helped debug this.
It seems that the 10.8.5 update and/or new NVIDIA driver have... drum roll ...fixed the KP problem with two GT120 graphics cards on a Mac Pro. That has been the case with mine, at least. Using Safari in two monitors with multiple tabs and lots of browsing has been entirely uneventful this past week. I couldn't go a half-day without a KP before this with similar use.