Low light performance is tricky, because beyond f2 the trade off in DOF is so great that it often makes sense to just bump the ISO.
That said, investing in the latest and greatest body (for ISO) is not really a sound investment either, because they become obsolete pretty quickly.
Since you are used to using your 50 1.8 on film, I would suspect you would see similar low-light performance with a K-X and DA L 35mm f2.4, but do know that you will need to have higher ISO and you won't have as thin a DOF.
The difference between the Nikon and Pentax lenses as far as DOF go are not huge. It would be prudent to compare the costs of the full lens lineup that you intend to buy. It will give you a better idea of which camera system to buy into.
Overall, it could be said that Pentax makes high-quality lenses that are not particularly fast, at very competitive prices. Nikon has cheaper lenses, and far more expensive lenses. In general, you have to pay more with Nikon for quality, but you can find some really cheap consumer options in there as well. Nikon has a better autofocus system. Pentax has better backwards compatibility, and in-body shake reduction.
Pick your poison
. And buy a flash.