If you're only using it for sports, don't bother with the OS version. Find the older Sigma APO 70-200mm f/2.8 Macro HSM, it focuses mad fast, super bright at f/2.8, and pretty darn sharp. It's not amazingly sharp (and a little soft at 200mm f/2.8), but it's super awesome stopped down a little and overall an awesome lens. Stabilization isn't necessary for sports. The older one goes for around $650-700 in good condition, but I think the lens is catching on, going up in price recently. Which sucks, I sort of want one I just have to accomplish some other purchases first.
People look down on Sigma a bit, but my 24-70 2.8 (the older non-HSM version) is crazy awesome sharp at any reasonable aperture, still solid at 2.8, built very well... pretty much professional level stuff except for the lack of weather sealing. The 70-200 I used in high school so frequently (I've mentioned it in way too many posts on this very forum..) was one of my favorite experiences ever. I liked it just as much as the Canon 70-200mm f/2.8 and 2.0x TC I rented for a surfing trip in hawaii.
Check out this review of the lens:
Sigma AF 70-200mm f/2.8 EX DG HSM APO - Review / Lab Test Report
And curveball, and recently getting more and more expensive, but look at the Sigma 100-300mm f/4.0 HSM. If you don't NEED the 2.8, it's even better optically, and gets you a little extra reach.
Here's a review of that lens:
Sigma AF 100-300mm f/4 EX HSM APO - Review / Lab Test Report
Q: What body are you using? I'm assuming a crop body, so I picked those reviews.