At this point, I am utilizing three outputs to three 24" monitors.
I would like to add an additional three monitors to this configuration and need to know everyone's opinion on the best course of action.
The motherboard has three PCI Express ports 8x8x4.
Could I continue to use the two cards with SLI and simply add an additional card to utilize all six monitors at 1920x1080? Is SLI required for multi-monitor setups?
Basically, I really just want to be able to have several windows open on different screens for web design, virtual machines, etc. I do not plan on gaming... Unless there is an alternative card configuration solution that would be cost effective.
I think that you'll have to update your graphics to get more displays. Some AMD cards can run six displays on their own and all of them can run at least three AFAIK, so it wouldn't be an issue to get six displays with however much performance you want. I you want to stick with Nvidia, then you're practically stuck with needing two cards to get six displays AFAIK, still not an issue.
Upgrading your PSU won't be necessary if you don't go way overkill in your upgrade, but it wouldn't be unreasonable.
If you want strictly Nvidia suggestions, I think that two GTX 650 cards if you want an upgrade in performance over your current cards. They can be had for around $200-250 altogether. More of an upgrade would be two GTX 650 Ti cards if you want more performance.
It seems that the supply of cheap AMD cards with six display outputs has been exhausted. The best that I can find support up to four display and there are a few options from both AMD and Nvidia with such functionality.