Maybe the DVI to VGA adaptor is bad. This must be a old monitor if it still has a vga cable. Know anyone with a DVI monitor you could try? If that doesn't work, put the old card back in and try updating your bios.
But even if the adapter was bad, that doesn't explain why the computer wouldn't boot or load up the OS. (Only when the XFX card is installed; the computer boots up fine when I take the XFX card out.)