XDR has a lower pin count per equal throughput compared to alternatives.
It is similar to PCI Express in many respects (Read up on why PCI Express is better than PCI, and I am talking about more than just clock speed and bus width here).
It would permit cheap to manufacture memory, with a 256 bit interface (possible / cost-effective on most PCBs) it would provide 'worlds' of throughput.
As for Cell being good gaming, most people already knew that ages ago, whatever does the math (including physics), AI / logic, and moves the data (eg: textures, audio, etc) around the 'best' way will be the 'best' for gaming.
Only problem is coding well for a given platform, and compiling and testing said software (games) once coded. That might be the downfall area.
Lack of decent software studio / development tools / advertising said tools / training & documents of said tools / etc is a very common factor in architectures failing, and often overlooked. (Just look at the entire history of computing).
The next 'major' issue (for x86/x64) is going to be the ever increasing space that advanced Out of Order (OoO) execution units require to keep scaling at the same rate. (Eventually they'll require the same amount of space as L1/L2 caches do now, and then the multi-core designs will not scale so nicely.... unless they trade OoO complexity for sheer number of cores or better performance in other areas. eg: FPU, or number of FPUs)......
..... which is exactly what IBM Cell is doing ..... and to some extent what Intel are doing with IA-64.
Even with constant de shrinks x86/x64 is heading into a brickwall where it can no longer scale performance like it used to..... or rather it will not scale performance anywhere near aswell as other designs..... (Looking at IBM Cell, Sun Microsystems UltraSPARCs, and Intel IA-64 here).
What I want to know is: What do AMD have planned past 2012 ?, as x64 will hit a wall then.