The above bikes are great choices but also check out the KHS XC604. It is a really nice bike for the money. A couple of people around here are riding them and they are very pleased with them so far. You could definately enter it in a race or two. You should be able to get it for under $1250 so you will have some extra cash for some upgrades or race fees.
http://www.khsbicycles.com/02_xc604.htm