Bob, in point of fact, the Pentium 4 package is actually smaller than the Xeon package, but that's not why there aren't any dual Pentium 4 motherboards out there. The Pentium 4 and Xeon are effectively the same processor die, except they differ in operating margins, as well as packaging. Intel makes it so that the Xeon is better suited to withstand the electricals of a dual processor system, and validates them additionally to ensure this. (Xeon MP is a different processor die, though, because it has the extra cache on die to boost performance.)
For Hyperthreading, I think there will be a definite advantage for many different users. For server environments, you might be right that adding a second physical processor is still better for throughput than allowing the second logical processor thread to compensate. For desktops, most users make use of the Windows multi-tasking environment, which is accelerated by Hyperthreading. Once more applications get optimized for Hyperthreading, I think we'll see a lot of individual applications get a performance boost as well.
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