Sorry about the poorly-worded title.
If I want to output to four monitors with a dual 8800GTX setup, can I expect normal behaviour and performance when spanning a window across all four displays at once?
Will MAX_TEXTURE_SIZE be increased to 8192, so FBOs will be usable at greater than 4096x4096?
The TripleHead2Go offers such poor resolution that I don’t think it’s a option for me.
If you create 2 windows each one spanning 2 displays connected to 1 GPU, I guess then it should be just fine, since this functionality is already supported with 7800 GTX
Thank you, but I already know that two windows will work. 
From an ease-of-use perspective, I need to know if I can just create a single window across all the monitors, and have things work.
Knowing if the viewport/texture size is now 8192 is also important for the same reason.
I believe it does not depend on texture size since they are two different hardware devices and you are talking about using them in SLI compatibility mode. Yeah I agree it would be helpfull to me also to have that functionality without having to deal with multiple rendering contexts and switching between them.
That’s probably not the optimal approach.
You’re taling about rendering to a huge viewport, even if it works, huge viewports can have unseen consequences for example guard band size, clipping and every SLI mode has a tradeoff, e.g. cache coherency.
Texture size is not tied to viewport size in any way unless unless you have some unstated requirement.