You asked
Is the pentax k x good in low light?
The site doesn't rate the K-x's high-ISO performance, so I can't tell you how clean it is at, say, ISO 3200. It's a 12.4-megapixel APS-C camera from 2009. A full-frame body gets roughly a stop of advantage in low light at the same framing and f-number, and the K-x's in-body stabilisation helps when you shoot handheld in dim rooms.
The lens often matters more for low light than the body. These fast primes all fit the K-x:
For the reasoning, see Sensor sizes. The full lens list for the camera is on the Pentax K-x page.
Written by Claude from this site's pages.