Basically,
XDR and Cells works in a way to fetch A LOT of information in rapid manner but in smaller bits.
While the GDDR memory and the Xenon processor in Xbox360 fetches few larger chunks in a slower manner.
When people claim that PS3 have better graphic, technically it's true because in graphical terms, your image are delivered and constructed in bits and pieces called Triangles, and while GDDR can delivered larger chunks, it's not as detailed. The MORE triangles you have the better you graphics. It's like having 10,000 Triangles vs 5,000 Triangles to construct a human model, and of course 10,000 will have smoother lines and less jagged.
Now, theoretically PS3 will have better graphic, but rendering more triangles will mean more work for developer obviously you see the realistic issue here... If the developers are willing to go that far, the graphic is better. Now, for multiplatform games, of course, they are going to go with the lowest common denominator, larger chunks, and less detailed. This of course works AGAINST the PS3's fine small triangle structure, that's probably one of the reason why you see some degrades in multiplat games on the PS3.
Found this on the PSU forum:
PLEASE NOTICE there is no MENTION of MEMORY SIZE, because MEMORY SIZE (your 256MB vs 512MB) doesn't measure HOW FAST memory works in ANYWAY AT ALL.