I'm looking at getting a Nikon DSLR-v (likely the predessor of the D700 if it has video or the D3s) and I'm wondering what add-ons' people are using to make it more like a video camera (like can you use those external screens & mics?)

I use a Rode Stereo VideoMic on my Pentax K-7 that external mic port and HDMI output for external LCD monitor.
http://i8.photobucket.com/albums/a21/jogiba/Pentax%20K-7/screens1of1-5.jpg

Does the external LCD do real time video off the DSLR?

Yes, it's the same image you would see on your Live View LCD on your DSLR with Live View and HDMI output.

http://i8.photobucket.com/albums/a21/jogiba/Pentax%20K-7/screens1of1-67.jpg

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Excellent info Joe. That will be very useful and I'll add it to my budget.