Yeah the thinking was around E3 last year that the GPU going into the xbox360 wasent as completly revolutionary as ATI kept claiming and was just an beefed up x1800xtx with some new features. Given the speed that ATI got the x1900 given much of their development team being tied up with the x1800 problems its assumable that it was a spin off of the xbox360 project. I dont think ATI have the resources to be workiing on 3 GPU's at once.
And i don't think the the whole unified shader architecture does all that much, i just think it can help out in some odd unbalanced situations where the current design doesnt do all that bad to begin with. The dedicated ram to do antialiasing is something unusual though, but if it were so great then you would have thought ATI would be quick to stick in on their PC cards. Im guessing that at 10mb it only at relatively low resolutions. Also i would think the low speed of the GPU (thermal issues?) holds back performance a bit, given the speeds of other ATI gpu's
CPU wise i dont know, but devlopers have said it isnt as good as an Athlon 64, so it has to be holding any top notch graphics card back, especially at low resolutions.
I would think all an Dual Core Athlon/Opteron, 2gb of Ram and a x1900xtx would wipe the floor with an xbox360. Just think then, what a pc equipped with an FX-60, crossfired x1900's (or 7900gtx's) and 2gb of ram all overclocked with some sort of RAID array. That would really put the Xbox360 to shame. And with new things coming, like Conroe, DX10 cards, extremely fast DDR2 modules and a hyrbid hard drive, the gap will only widen.