Lens Recommendation

I have a Pentax *Ist DL and used the bundled 18-55 zoom lens almost exclusively this year. I was happy with the results, but not totally satisfied with the sharpness, contrast and at times, color. (as compared to some of the photos displayed here and elsewhere) I know the color part is probably my settings.

I am looking for a really good quality lens to take the photos to the next level. The track I normally photograph at allows me all access to the track so the need for a longer lens is un-necessary, in fact my 18-55 gave me great close-ups.

Any particular brands? price ranges?

I have been looking at the Sigma lens but I am not certain whether it will give me higher quality than the Pentax lens, are there any other quality makers left out there?

Thanks, I looked into the Sigma lens, I really like it and if it performs as you say then it looks like I will have to get really creative in making some xtra $$$ to be able to afford that lens.