5 monitor configuration question

I'd go for the third option, given that the PCI card won't be required to drive any complicated graphics, just mere "spreadsheets" like you said. Hopefully someone else can shed more light on improving video performance on CAD, but I know it shouldn't be as demanding as hardcore video gaming, unless you have to do 3D renders on a regular basis.

Yep, agree with r_m, simplest thing that does the job just fine is a cheap PCI card (would recommend a cheap nVidia GeForce6 series for driver compatibility) since you're not trying to do any heavy lifting on the card.

The WORST idea would be USB as it would do alot to steal CPU cycles and would be more of a pain to manage over IRQ issues and also from experience be laggy as heck.

There are 4 port Quadro cards, but depending on the current 2 port card you have in there it's almost guaranteed to seriously reduce your vertex pushing power.

As for the application itself, it depends on what it is, CAD-like doesn't really say much, and even in the CAD / modeling realm there are different requirements, where some benefit greatly from large amounts of memory to manipulate large wireframe models, layers, etc. others benefit from shader power to do vertex & geometry calculations.

Often if it's OGL based then the drivers will help the app enough that sticking to a workstation card is the best bet. D3D based apps can very often get huge boosts from the latest gaming cards.

But if you're happy with your current setup and how it handles the app, introducing a competent Geforce PCI card should be the best way to address the extra monitor.