I have one DVI and one VGA, the VGA connected through the minidisplay to vga connector isn't working.

Reference the article with the chart.

Also, pasted from a similar post:

I have a 2009 mac pro with an ati 1gb card. it has a dvi and two displayports. The dvi monitor has my dell on it, no problems. I had this old vga monitor, got a mini displayport to vga adapter, plugged up, and nothing. The monitor's power flashes as it appears to know it's plugged in, but nothing comes up in sys prefs or even system profiler. The monitor is pretty old i mean, probably around ten years, but it's just a vga. Any ideas?