Graphics card capabilities-Multiple monitors

So, I have a Dell XPS15 with an i7-2720QM (which probably isn't relevant for this, but hey) an Intel(R) HD Graphics Family (onboard graphics card) and NVIDIA GeForce GT540M graphics card.

I have a minidisplay port, HDMI, USB3, and eSATA ports to choose from.

The laptop's display is 1920x1080

So I have a few questions, most fundamentally what are the capabilities of those graphics cards/my computer?
Can they run 3 monitors simultaneously? (Can the onboard run my laptop's built in monitor and the GT540M run externals)
What really happens when you choose to run multiple monitors (in my case, I guess) Does it allocate a graphics card to each monitor or does it pick one graphics card and multitask that one on all the monitors that are running.

More practically and on to the monitor side of things: What really matters in terms of the amount of "space" a monitor gives is how many pixels it has, correct? Having a 30 inch 720 monitor won't give as much screen "space" as a 21" 1080 monitor, correct?
With that in mind, how large a monitor, in pixels, can my current card setup support?

Also, if I felt like it could I potentially drive 2 large monitors on my desk (not driving my laptop's built in monitor) using, say, the HDMI and minidisplay port at the same time?

Sorry it was long, but these are things I want to know and I've had a hard time finding the answers myself.

mini display port can push onto multiple monitors or at least is capable of it providing the hardware is there for it, ati do support that on there eyefinity cards.

you may be able to run your laptop monitor and 2 external monitors over hdmi and minidisplay port but it all depends on whether the graphics card actually supports this.

i can't find any direct evidence to support 3 monitors on that gpu but i'm still looking.

also due to the fact it's a laptop and the onboard graphics and the gpu share the same display ports unlike a desktop the chances of utilising each for different displays is slim to none.

oh and if you connect 2 desktop displays then you should be able to disable the laptop output in either the nvidia drivers or using windows personalisation page>display>screen resolution.

Thanks, if you find specifics on the GPU let me know... I can't seem to find it anywhere.