I think Nikon was following the traditional film speed with the D50. I know you can push or pull film but then you have to use the whole roll that way but for the most part we used 800 or 1600 there was no in between that was readily available. Plus if the D50 had the same specs why would you buy a D70s for 200 hundred more. Friends of mine who have the D70s would rather the ISO steps not be 1/3 because it makes manual ISO changes a pain by stepping up by thirds.
Also the D50's software and ISO settings are optimized to reduce noise at high ISO's ie 800 and 1600 which according to several reviewers and Nikon is lower than the D70s this may be some of the reason for the bigger steps but I think its marketing to keep a demarkation between the D50 and D70s.
But if the ISO steps are to big and really bother you then a D70s is a good choice and the learning curve will be short. Good luck but if you sell try here at nikonians first if you upgrade to silver I am sure there would be several film buyers that are Digital cutious but do not want to drop a boat load of cash on a digital body.
ISO increments on digital cameras are almost always one stop increments; 50, 100, 200, 400, 800, 1600, 3200, ... . Smaller increments, while possible, would be finer adjustments than necessary.
BTW, one stop increments in ISO and shutter speeds are double or half of the previous value so the increment between 800 and 1600 is the same as the increment between 100 and 200. Its a geometric progression and not a linear scale. With f/stops you multiply or divide by the square root of 2 (~1.414) to get one stop increments.
All reviews to date find that the camera is pushing the limits of the higher ISOs and it has even better high ISO performance than the D70, so that should be some comfort. If you need more detailed control, it's really time to consider moving up to the D70 or the D200.