Henrik, about those mountains. I made them to fit better in with the highland hills, so I think you will get a good result if you mix them a little in the andes, and perhaps add in a few snowcapped ones. I've made some snowcapped mountains for special cases (I use them as barb units too).
Also you can manipulate a little with the last terrain square, to get a more flat look. The important thing is that you still cover the "greens" of the 2-3 squares behind the central square, if you use this idea. But you should be able to make it considerably more 'flat' if you wish, by lightening up the dark shades a bit. Alternatively, make the central square totally flat, and grey out the ones behind it.
BTW, I've made a new set of jungles. But I'm not sure I'm totally satisfied with it yet. The 'blocky' thing then comes primarily from the base sqare, which can then be changed, if one wishes.
Prometeus : looking good so far. I notice you've been looking deep in the civ3 art folders too.

I made an abbey terrain square once, but was not completely satisfied with it. Can you post a map shot with your terrains on it, along with the city graphics etc?