I have a Quadra 700 accessory kit with shrinkwrapped manuals, disks, registration papers, and a shrinkwrapped Apple ADB Mouse I. Should I use it with my upcoming Quadra 700, or keep it's contents in their shrinkwrap?

Keep it, definitely! What's in there that you can't easily find elsewhere, eg there's ADB I mice everywhere, but a still-shrinkwrapped and pristine one is truly special! (in my opinion)
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Main system: custom C2D 2.66GHz tower/4GB RAM/2600XT 512 MB/500GB HD/XP Pro x86+Vista Business x64
Main Mac: Beige G3 minitower (Sonnet G4 500MHz/640 MB DVD/USB/Radeon 32 MB/OS 9.2.2+10.4.11)

I sort of feel like I have to open it. I bought it
because I wanted to use it. I probably will end up using it.

That mouse would go well with the NIB Apple Extended Keyboard II I opened and used. It would also go well with my Quadra 700 hooked up to a 20" CRT monitor.

Well ok then, I won't open it. I can already feel the guilt of if I opened it.
I will open the microphone because I often see NIB microphones. Is that ok? And can I use the system disks?

I won't open anything because it will be nice to have something special. I'll just look at the pretty box. (seriously)

I say copy everything that's not shrink-wrapped and make a copy to use. Save the originals. And keep that mouse in its bag!
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PPC Macs:

• Power Macintosh G4
  
• B&W Power Macintosh G4
  
• Power Macintosh 6500/225

I don't need copys of the stuff. I'll just use System 7.5.0 when I get a Quadra 700, instead.

I will. I found an extra mouse, so I don't need the special mouse anyways.