great start! Just remember that in most cases a prime lens is going to outperform a zoom in image quality in a number of ways. There is a trade off between having the flexiblity of a zoom and the image qualities and speed of a prime lens. The FA50 is an awesome lens and is faster and delivers great images but it's fixed. The zoom will be slower (needs more light) and will not be as sharp as the prime. Both have a place in the camera bag though. Everyone from hobbiests to pros will shoot with both types of lenses.
To see some examples from each have a look at these images:
50-200mm
PENTAX Photo Gallery
FA50
PENTAX Photo Gallery
BTW the Sigma is a great lens