You want to assamble the TAMIYA twin-motor gearbox in "C"-mode, ratio 203:1
The A & B setups, 58:1 are really fun because they are fast etc.. until you try to make your robot use it. It is simply not strong enough for the fast-changing pulses etc from a PIC-controller, it is useless because it takes so much time to accelerate, and many setups will not ever get up to speed at all..
I am sure you do not believe me (I would not), and I am sure you will have fun assambling, testing-deassambling, realizing the new hole does not fit with your creation, re-building etc etc ;D
I did do the C configuration, before I read your post. Thought that the 58:1 was not enough.
Speaking of Polulu, they have another double-gear box: http://www.pololu.com/catalog/product/114 which can be configured to 115:1, if you wan't something inbetween the 58 and 203 ratio.
For the hill-climbing workhorse robot, you can go for the http://www.pololu.com/catalog/product/68, which can be configured to 5402:1.
... And the box is sitting next to me... at work... can't wait to get home.
I was tinkering a bit with electronics some years ago, but haven't touched it a while. The robotic thing is a bit of an excuse to take up that hobby again.
The first plan is to just create a small robot on wheels or the tracks. I think I'll mount some bump sensors on them at first. Later, I'll try the "LED as a light sensor" project as sensors for a line follower. Also, since I forgot to order the ultra sound sensor, I have bought some transducers and plan to see if I can create my own sensor. It's fun challenge and you learn more about the electronics by creating your own stuff. If I succeed, I'll put up a guide here.
I can't remember if I wrote it anywhere, but I did manage to get a small Lego (oh no. Frits is going to yell at me for using the L-word ;) ) car going straight with the adder/subtracter or whatever it was called, that you described. I had some difficulties, because my differential gears are the old style, that are larger than the ones you used. I don't know if I will finish it, now that I have the other stuff to build from, but maybe. After all, the Toy Which Shall Not Be Named is fast for prototyping.
Oh, and I got a solar panel (or solar cell or whatever it's called in English). I'll try to see if it can deliver enough power to drive a small robot. It's rather large (http://www.sparkfun.com/commerce/product_info.php?products_id=7840), so the robot can't be that small. On the other hand, there is a limit to the power it can deliver, which put a limit to the weight of the robot. It's going to be a fun challenge.
It must feel almost like christmas! :-)
Do you have any overall plan on what you're going to build and what it will be able to do or are you just going to experiment a little with the new parts first?
From your previous postings it seems you know stuff about electronics... is this motivated by robotics or is it a hobby you have or something?