First of all which graphics cards support 3 monitors?
Is it possible to buy a graphics card which has 2 outputs and use the onboard (motherboard) graphics output for the third monitor?
(as far as I know you have to toggle between onboard or external graphics usage in the BIOS)
You should consider Eye Infinity from AMD/ATI with screens that support DisplayPort :
http://sites.amd.com/us/underground/products/eyefinity/Pages/eyefinity.aspx
And, I don't know if Matrox still make hardware but if so, they have solution that allow you to do this.
That said, I think a EyeInfinity video card will be much more cost effective than Matrox stuff, please correct me if I am wrong on this.
And about using the onboard VGA, I don't think it's possible since the onbard VGA is automaticaly disable when something is plugged on the PCI-E port.
Regarding the second question: Yes.
We have a bunch of older Dells (like Dimension 3100) at the office that can run 3 displays with the addition of a video card. 2 from the card and the third from the onboard video.
Yes, EyeFinity from AMD/ATI. Supports up to 3 monitors.
But if you're getting a card, check whether the ports on the card will fit your monitors. My 5770 here has a Display Port, HDMI and 2xDL-DVI. I forgot when I was buying my third monitor (which only outputs DVI as my other 2 does) and so I had to forgo my oldest monitor, 17", to get my new 22" on.
Of course, I could've gotten a converter but I figured the 22" widescreen had enough screen area to make up for the 17" :D
Also see my answer for a similar question, particularly the USB video adapter comments: Multiple Monitors
