Can I run 2 video cards (1) NVIDIA GeForce GTX 660, main slot, and (1) GT120 in a 2009 Mac Pro? Reason being, every time there's an OS X update, I have to install the GT120 being that the GTX 660 does not have the normal boot up screen. Not that it's a big deal but if the Mac can handle my 120, why not keep it installed.. The big question.. Are there an downsides, sluggishness, crashes, etc? Also, I probably won't have any monitors plugged into the 120 as both of which I have now, are plugged into the 660. Thanks!