3 Monitors to a single pc

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.

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.

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