Not sure about Olympus E420, but being a Nikon User (D90), I can comment on the Nikon.
First of all, the D90 and the D80 is closely related, with nearly similar specifications. ISO control in the Nikon is good, meaning you can take clearer pictures at low light situations and at night without noise. With Nikon, you can also gain access to a wide range of lenses, starting kit lens for the D80 is 18-135mm without VR.
Taking structures and people, you will usually use different lenses. Maybe for structures, the kit lens is good enough. For people, you can get an additional 50mm f/1.8D which is around $180-200 new. Or better still get Nikon's latest AFS 35mm. So might be good to get a Nikon for expanded lens variety.