Pick up d Nikon 20/4 to test tomm in daylight. D 21/4 CV cannot make it. Haha.
Here's a resource I found useful the Olympus 21mm f/3.5 is not expensive and images I've seen from it looked good: http://www.16-9.net/lens_tests/best19_21.html
Going the Nikon route, your best bet would probably be the 20mm f/2.8 AIS, short of the higher-priced (and not much better) 18mm f/3.5. Don't pixel-peep on any of the Nikon wides ;p