Also, the XT, might have 320 stream processors, but they run at a lower speed, than the GTS's 96 stream processors. Actually, I'm not sure about that. Maybe that applies to, GTX's stream processors. Somebody please, correct me, if I'm wrong.
GTX stream processors operate at 1.35GHz
GTS stream processors operate at 1.2GHz
XT stream processors operate at 743MHz
Still HD2900XT is the best out of these in terms of shader operations. However, 8800 series have higher pixel fill rate and texture fill rate because they have more Raster Operators and Texture Units