I've found that with a dual head card, I can have two monitors (the DVI connection needing the adaptor). I can extend this further, using a double adaptor to create a third display but that makes only a copy of whichever output it's on (useful only for remote viewing).
As long as your laptop has two outputs you should be able to run your two monitors with the lid closed. The primary screen (on analogue) will be a copy of the machine's built-in display and the other (on DVI/HDMI) will be a desktop extension.
If it only has the one output, then you'll be limted to just the one extra monitor. I don't use Macs but certainly on Windows, I have a choice as to whether to set it as primary or secondary, thus extending the screen area onto the second display. On my laptop, I have to use the inbuilt display in conjuntion with the external one, to get the extra space.
I have no idea how you set it up on the Mac OS, but I'm guessing it will be similar.
Does this help at all?
Dr. V
