I am not an expert in hard-core hardware, but the people at the German Magazine CT are famous for knowing that kind of stuff inside out. Today they published a story that analyses the impressive hardware specifications of the Microsoft Xbox 360. They take apart the rather marketing flavored numbers from the published Xbox 360 specification and break them down to what they really mean.
For instance the claimed theoretical 1TFLOP/s performance of the Xbox 360 is not measured in a way to be acceptable for the Top-500 list of the most powerful computers.
The more serious issue the Heise article raises is about the graphics performance at HDTV resolution (1920 × 1080px). At this resolution with 32bit color depth and a 4x multi-sampling for anti-aliasing the Xbox 360 frame-buffer of 10MB is way too small. To compensate that the transfer-rate to the GDDR3 is important, and that is only middle-class compared to PC graphic-cards.
Read the full details on Heise (German). (Google Translation)