It doesn't matter whether the cores on the 360 are on par with Intel. It doesn't matter as the data code to decode h264, VC1, etc is run in tight loops and doesn't need to branch. So decoding VC1 suits processors which can run at incredible speed at the expense of branch prediction. And the Xenon and Cell are absolutely brilliant at that. And this also happens to be what the Intel and AMD chips are not execellent at. Sure SSE2/3 and Altivec will help, they just have to get the decoding working first before they start optimising that code.
Nobody has yet managed to get VC1 decoding on a PC at anything above 720p. Though even at that rez, they are constantly dropping frames.