Based on "Experiment_626's" experience, you might just get the Canon 70-200 f/4L for that
Amazon price. If you want used, a used one on the Canon forums run about $490-$500. Personally, you might as well get new. The IQ of the "L" lens will help tremondously when you are shooting at ISO 1600. It'll make for easier clean-up in post-processing as well (Photoshop, Bibble, Paintshop, etc...)
A monopod might help, but I think setting the shutterspeed to at least 1/500 will do wonders.
There is absolutely no question that a Canon "L" lens will outperform almost all third party lenses, especially a Quantaray brand which is arguably a Sigma consumer line. If you have a local store, I highly recommend going to one and trying out a few lenses. For me, switching from the Quantaray to the Canon "L" lens was like a night and day difference. IQ, Faster AF acquisition, etc...
Another thing is resale value. I tend to move through lenses as I get better. I sell off the older ones to help fund newer ones.
Imagine my surprise when I tried to sell my Quantaray on ebay or even on the Canon forums....the prices were so low (around $50) that I just kept it. The Canon 70-200 will last you forever and if you wish to sell it, you'll get decent resell value. The same deal w/ the Sigma EX series, and the Tamron 28-75 f/2.8 and 17-50 f/2.8. If you do a lot of reading on the Canon forums, you'll soon see which ones are the highly demanded lenses....and many are good values as well.
www.photography-on-the.net/forum is a good resource to check out lenses. I'd recommend posting your question there as well. There are many pro sports photographers on there that can give you much better insight than some of us on this board.
Good luck w/ your search.