OK, some generic answers:
Having 2 displays should not create any more devices, as far as X in concerned. There are loads of threads already about using multiple displays...just have a quick search, but be prepared to edit your XF86Config file manually, I don't think there are (m)any programs around that'll do it for you.
Having two sound devices...I tried it once, but didn't work...but then, they were both ISA devices, and only one was PNP. If they're both PCI cards, and you know that they both work fine alone (i.e. with only one in the machine at once), then you could be lucky. You'd probably get some new devices, [/i]/dev/dsp?[/i] being the one that springs to mind (if they're newish cards).
How do you redirect system sounds? I think, under KDE, you can select which output device (/dev/dsp or /dev/dsp2) to output system sounds...have a look, I can't remember.
I'm affraid that's all I can think of, at the moment. Most of your multiple display questions should already be answered (in one way, or another) on this site. Good luck, and have fun.