D40 & D90 High Sensitivity Comparison
I took both cameras for a drive and here are the comparison shots. Even if both have the same sensor size, the high pixel density of D90's sensor still kicks D40's butt...
Nikon D40 and D90 ISO 1600 Comparison
(click on image for bigger version; wrong text input for D90 side: it should have been D90 ISO1600 f2.8 1/10s)
Nikon D40 and D90 ISO 3200 Comparison
(click on image for bigger version; wrong text input for D40 side: it should have been D40 ISO3200 f2.8 1/10s)
Both images above were taken in NEF and post-processing (via PS CS3) was identical.
But once weather cooperates (hopefully soon), I suspect that D40 still reign supremes over D90 in infrared photography (using a Hoya R72 IR filter). Well, let's see.