Triple display - realistic requirements?

I'm planning a new system build to take advantage of newly aquired monitors. Primary function will be software development, but I'll be damned if I have a multi-monitor system incapable of gaming. The point of me making this thread is to get some of my questions answered and also establish a budget that I should allocate for graphics with the new build.

We have a total of 5 monitors. 3 19" fullscreen HannsG JC-199DPB's and 2 17" widescreen eMachine E17T4W's. I plan to run the 3 HannsG displays through a Matrox TripleHead2Go to attempt to take advantage of the 2 17" displays without requiring a third video card (and also to take advantage of the neat TH2GO features). As such, the display setup will *technically* be 3 monitors. We would have the 2 17" monitors, at 1280x720 resolution each as independent displays, and then the 3 19" monitors as 1 monitor via the TH2GO coming in at 3840x1024.

The goal is to be able to do day-to-day development on all 5 displays, while reserving the capability of gaming on the "single" 3840x1024 display.

What would be the best way to accomplish this? I have not decided on any hardware for the build, as the 5 display goal dictates everything.

My current line of thinking is this:
Dual SLI capable graphics cards with dual DVI output each. Primary card outputs to the Matrox TH2GO. Secondary card output to the 2 17" displays. In non-SLI mode the system should behave as a standard triple-monitor setup. In SLI mode, obviously I'll lose the 2 17" displays.

Anything I'm missing, or not thinking of? Anyone try something similar?

I'm not a balls-to-the-wall gamer, but I would like to be able to play a game like UT3 in 3840x1024. I generally keep detail settings on the low end of the spectrum, but it would be neat to have something capable of more.

On to the specific questions:

1) Is an SLI setup the ideal way to go to render effectively on the 3840x1024 display? Dual SLI, Triple SLI, what?
2) If I go SLI, am I correct to assume any cards capable of rendering the 3840x1024 effectively would be more than enough in day-to-day desktop work?
3) When dealing with multiple monitors and multiple video cards. Is it true that the primary card (talking non-SLI here) does all the work, and simply outputs its work to the secondary video card? Or is the secondary video card doing some of the rendering as well? Is this the same for a DirectX and 2d environment? For example, if I wanted to save a bunch of money could I get by with a single modern PCIEx16 card as primary and some low end PCI card as secondary?
4) Would I, perhaps, be better off going with 3 video cards and no TH2GO? Maybe 2 modern cards and a run of the mill PCI card? ie: 2 monitors on primary card, 1 monitor on secondary card, 2 monitors on PCI card. Or 2 on primary, 2 on secondary, 1 on PCI...? It occured to me that a third, PCI, graphics card would be cheaper than going with the TH2GO system. At this point we'd be diving the 3840x1024 between two cards.
5) Lastly, what kind of budget would you experience hardware gurus establish for the graphics card portion (ie sans TH2GO price) of this project?