Normally I would be on the lens side, however there is a balance between the combination of the body and lens in photography. The 18-55 kit lens is pretty good, and yes its f3.5. Moving to a faster lens, is a possibility, however it will be much more expensive, and physically larger and heavier. I doubt that f1.5 is in the cards, probably more in the f2.8 range. That said, it all depends on the focal length that you are looking for. The 16-50/f2.8 lens is a chunk of change, and larger to boot. The Tamron 18-200mm F3.5-6.3 is still in the same aperture range as the kit lens, and when coupled with your K10 body, will still have the same light performance.
On the body side, now may be the time to look at a body change/upgrade. Why, it comes down to ISO - sensor sensitivity. The K10 is great - in the low ISO range - the color is specular (with the CCD sensor). However, as the ISO increases, the noise also increases - and hence your question. With the K5 being cycled out, the prices are down and the ISO is exceptional in the higher ranges. With a higher cleaner ISO, your current lenses would be much more effective in the lower light situations that you are concerned with. The K5, K30 or even the K-01 - you have an excellent selection with out paying a premium.
Photography is all about finding a reasonable compromise - a balance across all the components. There are times for lenses, however - there are also times of bodies...