Fast lens for Pentax K100D?

I have the Pentax K100D and am looking for a relatively fast zoom lens
(f2.8 or better throughout) to take indoor sports photos. I'm getting ok
photos with my kit lenses (after NN and PSE3, of course), but am hoping
for even better.

I'm thinking something in the 18-50mm or 18-70mm range, maybe, to
replace the smaller kit lens (18-55mm f3.5-5.6) I have. I'm not sure
what, though.

I found this one on eBay - what do you think of it? Worth the $$?
Sigma 18-50mm EX DC Digital Lens for Pentax K10D K110D f2.8 throughout.
http://tinyurl.com/ypt79y
This auction has ended, but I'm sure I can find another one, eventually.

If not this one, do you have any suggestions on a decent lens (not too
much $$ - maybe around this price range or less) for me? I do want Auto,
though - am I being too picky for my price range?

I was looking for Pentax, Sigma and Tamaron. Any other mfrs I should
consider, or definitely NOT consider?

TIA,

--
Eric Babula
Milwaukee, Wisconsin, USA