Yes you can use two video cards. Your BIOS will let you set which to pick up first (Primary VGA is AGP/PCI), then you'll see the booting up stuff on that one. Windows' Display control panel then shows you the two monitors next to each other, you can enable the second one for the Windows desktop. Works great to spread your Windows desktop over two monitors.
But: most games only look at the primary video card. You are talking about switching between them, using your MX for some games and your V3 for others. You're going to spend a lot of time in your BIOS setup screens, and Windows will keep disabling screens on you, and/or defaulting the desktop to 640x480 16-colour, either way causing extra reboots each time you change over. If you only have one monitor it'll be even worse, you'll need a video switch too, or a lot of patience with those damned thumbscrews that hold monitor cables on.
Or I might be talking rubbish, I don't play games.