I think it is a gutsy move. I moved up from
Pentax K100D to K10D and now
Pentax K20D. In the process of going from K10D to K20D, I want to explore two systems with Pentax K10D and
Nikon D90.
After some thought, I can't justify two system and hence I abandon Nikon D90 and went for the upgrade from K10D to K20D. If I were you, I will try out Nikon D90 as I find it well suited for amateur needs plus more down the road with good lens when you can afford them. I am not mistaken, D90 shares the same sensor like D300 and for that, the price is actually relatively cheap to get -- the best in the latest with some compromise.
I think the D200 may be a better suited camera for you in looking into short-term time frame but it will plague you with peer pressures in upgrading from older camera to something better down the line. If you think D200 is the cheap way to get into Nikon, think twice. When you spend whatever amount of money, be it smaller like the investment in D200, or something more substantial as in D90 or event the best in asp-c with D300, you have to see both the optical and the emotional aspect in justifying the cost. Getting something cheap can be expensive if you find yourself selling it in 12 months. In that regard, you may find yourself just as happy with D40 if you think switching system with a FF camera is important to you.
I see your investment in
Pentax K100D with combo kit quite expensive. You could have swapped your K100D with K10D by selling your K100D body only for $250 and get your K10D used for $375.00. An upgrade fee of $125.00 is right on the budget. And to further get a better lens, you can sell your kit's combo and move up from there with prime lens. Swapping between systems is expensive exercise and there is always the caveat that you miss certain thing in a system.
And considering that you are a student, I truly hope the D200 can meet your demands on a budget and that you can find lens that are cheaper for a budget as in M42 lens and some older Nikon good glasses.
I am serious amateur who are happy with Pentax but there are times that I wish Pentax can improve quickly and decisively to catch up with Nikon and Canon such as AF system and noise control.
But lacking FF is NOT the reason that I think of D90 or Canon 50D. It is the inadequate development of AF from K10D to K20D that have me doubt what Pentax is thinking. Whatever system you are in, there are bound to be strength and weaknesses that one has to have some faith and compromise the weaknesses for its strengths.
The Pentax system in
K100D/K10D/K20D surpass my need as an amateur and I don't see your reasoning that it can't meet your demands. In all seriousness, you can't expect changing brand will make you a better or professional photographer up a notch. And it is dead wrong to believe that changing brand will save you money in the hobby. If you have not gotten any money out of your hobby, don't count on it that you can someday make big money to justify your current purchase. In other words, please consider not to pay for what you have not earned yet. Perhaps I am conservative thinking and I try to enjoy my current moments not thinking too far ahead, I find it quite odd a reason to switch brand based on professional status on FF camera availability.