Sigma 70-200 f2.8 is considered a very good lens as is the Tamron 28-75 f2.8. There are nuggets in the off brands. For any of the lenses you're considering ck this site: http://www.fredmiranda.com/reviews/
If a lens isn't reviewed there (and most of the really cheap ones aren't) I wouldn't even consider it.
It's expensive no matter what brand to get that extra stop especially going from f4 down to f2.8.
The 30D is a great body you'll need to invest in some reasonably good glass to go along with it.