Well I don't speak for AMD, Intel or Sun for that matter but I have some comments regarding the questions here.

I'm sure it possible to make a lower electrical power Ultrasparc with less cores or slower clock or make a higher compute power version with even more cores  or faster clock, If you wanted to model or make one yourself the chip design tools and specs to do it are here.

While The  UltraSparc T1 has 8 cores and 4 threads per core giving 32 hardware threads  and no other commercially available chip comes close to it. It is more important to think of the whole system it will used in like I blogged before here, So once you see the final product you can see the reasons why the other choices were made.

If your interested in the AMD space we've used the same design criteria, and as Marc Andreessen discussed here (the numbers are here) its the most cost efficient solution.

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