...And actually, if you install a video card you do potentially have to take memory from your PC to run the cards (despite the vid card having its own memory), so this can slow your system.
Jamesblue - I'd stick with VGA and not combine DVI & VGA but that's just my opinion. Many companies offer dual DVI or VGA outputs. You have one on your motherboard and then get a dual out video card... I reccomend staying away from DVI (you can have a DVI out, just convert down to VGA) because you may find it more trouble than its worth to mix digital and analog signals going to the monitors - especially when the motherboard VGA out is only analog.
Also - as a rule of thumb you get what you pay for... so be careful getting any old card that'll put a picture up on three monitors.
so if you use one of those devices, do they contain their own video memory, or will they be splitting the existing video memory from the card to which it is connected?
In other words, if you have a vid card with one output and 128 video memory, if you connect it to one of those boxes so you have have dual monitors, would each monitor then only be getting 64?
I tried a USB-to-VGA adapter and it gave me tons of system problems, flickering, and color distortion but the DVI version works perfectly. I use the screen for charts, video etc. at max resolution with no problems or side-effects. Setup was easy. Not sure about the technicals of memory use, but I run 3d games and such on my laptop screen with both external monitors plugged in and it seems to work fine.
I struggled with this issue for several months, I even bought a dock and a second video card to use with it, but that setup never worked properly. Maybe it's something to consider if you're on a desktop, but using multiple video cards with different sets of drivers sounds like a recipe for trouble.
For desktops, upgrading to a quad-head video card like the Matrox posted above should work as well.
