Vassil3427 is right. As long as you don't drastically change the power output, you should be fine.
A good "rule of thumb" is not to exceed 50% of the total power output of the engine on a stock transmission (the same holds true when it comes to building engine power -don't exceed 50% with all-stock components).
Things like nitrous can be devastating to an auto tranny, when used in abundance, as well as any kind of forced induction. To give you an idea of what I'm talking about, I have pictures of a powerglide that I had in my old drag car... the bellhousing is shattered to little pieces thanks to too much nitrous causing the torque converter to "balloon".