Hi Tim
you should be able to get 30"x20" (75cm x 50cm) out of the D70's NEF file.
Remember, this will also depend on what dpi you are printing.....200, 250, 300 (more, less ??).
Printing really big will require a great, crisp shot to start with & obviously the intended audience will be looking at it from a distance (feet/metres), not inches, like a 6x4 or 8x12.
I haven't as yet had any large prints done (bigger than 8x12" anyway), but I have a few saved as TIFF @ 30"x20" at 300dpi - for when I do end up getting them printed. In
PSCS2, I have made an action that upsizes the image in increments of 10% (ie. 110% each upsize) until the required resolution size is reached (which for a 30"x20" @ 300dpi is 9000px by 6000px). There is some conjecture that upsizing by 10% increments does not help gain any clarity etc.. (instead of upsizing straight from 3000x2000 to 9000x6000), but I have the action made now, so I just use it.
Hope this helps