Multiple GeForce GT 120 Graphics cards on Mac Pro

There use to - and still may be an issue - but I think it was in Snow Leopard - using two GT120s

Snow Leopard is not supported with security updates and patches or updates like to Safari, only Mountain Lion and above.

PC cards and video out ports are always tricky as to which ports will work, and sometimes flashing can alter that.

One ATI 5770 supports 3 displays. You could use GT120 and something else. Had to posted here? read threads, looked for ideas ahead of buying.

PC cards will not have any video out until the drivers are loaded after login so you can't see what is happening, but the OEM GT120 monitor can.

The hatter is right that at one point, multiple NVIDIA cards caused a problem. But that problem was Kernel Panics, not failure to operate.

You need to be certain the new card works by itself. To do that, install it in slot 1 as the only card, and be certain all the old functions work as you expect. Then you have known-good parts going forward into more complex testing.

When using multiple display, you must be certain you have completely eliminated Mirroring, which will limit resolutions to those that seem to be possible for both displays. You should have a separate blue box for each display, and none should overlap.

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Apple never did acknowledge that tbey goofed and dropped support.

You made it harder using non-Apple card. For best results check MACVIDCARDS as good vendor