Erik is right.
I have two prints from the D70 here (6MP) both are 1 Meter wide. One is a One-Megapixel-Crop, the other a shot taken in a pig's booth with nearly no light, Flash 12mm & SB-800 @ISO 1250
Both are wonderful and if I had known better at the time they would even look more perfect than they do.
What?
1) careful upscaling
2) Do not use a Laser but a pigmented ink printer.
Why?
Because the Laser (Lambda, Theta) draws the print by point and the points do not interfer much
The pigmented ink dots flow on the surface and the software manages the flow beforehand
by calibrating for the ink and the paper. So the dots from the up to 12 cartridges of different
basic and additional colors make a pointillistic work in which the small points flow and cover
uneven micro faults in the upscaled picture.
Much better for upscaled pictures.
If you have real 400 dpi go for a DURST Laser Machine
If you need to upscale go for a pigmented ink
Check out upscaling solutions. The development in that field
was huge during the last few years.