I have recently built a trading system for metastock and esignal. Here is my setup, I have agp ati aiw 7500. This is a single digital output card, hooked up to dell 18 inch lcd. 2nd card is pci ati 7500 which has dual output, one of them is digital, other is analog both are hooked to dell 18 inch lcd. There is definitely difference in picture quality, digital looks much better, but I don't think it would matter too much for trading purposes.
The installation is not tricky but remember insert one card at a time and install its driver and make it work before puting a another card. Winxp supports upto 8 displays. after 2nd card is installed just right click on desktop and choose properties and go to settings and you will see two or three monitors there you just have to enable each of them individually. I hope this helps.