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.

i think the basic thing you are misunderstanding that RAM =/= speed, CPU's process power = speed. In a traditional computer, having more RAM will "help" speed is because instead of using your Hard Drive as "RAM" (which you probably dont' notice, but it's there as Virtual Memory), you have RAM that are specifically designed to store randomly accessed piece of information. Those information are dumped once your computer shuts off.

RAM works similar to HDD in terms of memory, but the difference is, that stuff in your RAM sticks are much easier and A LOT faster to be accessed, while HDD have a mechanical bottleneck to it (Spinning of the actual disk) as well as an interface bottleneck. The bigger the RAM, the more information that can be stored in location that can be accessed quickly.

Now, with the cell structure, it doesn't work like that, as of the details I am not sure. But I am guessing that instead of having to store those bits and pieces in RAM like traditional computer, the cell's 6 SPE (available to game developers) can processed the information real time instead of having the stuff store in a certain location and they chose to use 256mb of XDR type ram instead of GDDR3 type rams which are found on X360 and Wiis

If you know the XDR and GDDR3, you'll first notice that XDR's focus on speed, and I think that's what the PS3's main focus when building the machine... It's to process information quickly, and not relying on the old formula of dumping pre-rendered textures into ram... this obviously help with FREQUENT and RAPID rendering as opposed to statics.

Please note that the graph that is shown above is not correct as it "beautifies" the situation for XDR. Currently, XDR ram can be as fast as GDDR5 ram. An actual comparison of both ram architectures is difficult as it depends on many factors (bus width/bus clock/"deepness of the XDR"). The main advantage of XDR is its lower power consumption (and possibly less data/address lines depending on implementation).
If you really want to know about XDR, got to the rambus website and read.

If you really want to know about cell architecture, go to IBM website (much of what errorrr wrote is more or less incorrect).

Do most people even comprehend these terms? Telling people to read their website is a good advice, if the website can put it in laymen terms lol...

Man people will never understand how the cell and the ps3 work, i really explain this over dozen times here on VGCHARTZ, and a few dozen more in other sites, and I'm not going to explain again I only wil say that the ps3 work very diferently than a PC or a 360 thats all tha you ned to know, and thats why is dificult to program for, the cell is a new technology that can procces grafis like a vieo card thats the only thing i will say.