Windows via Bootcamp says it is an XT. Steve Jobs on the other hand says its a pro. Who would you trust in that situation? Apple is not a company to downplay ANYTHING. Why market an iMac with an inferior product if it actually has a better card? I definitely think not. And I really dont trust Windows that much.
All that really matters though are the benchmarks. You can check out the 2600xt post for more info on that. Or proceed to barefeats.com.