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I'm sure this is mostly concerning the recent Athlon64, as it is the first 64 bit x86 chip that I have heard of.
The benefit with it isn't 64 bit integers, as stated above it's that it has 16 registers per unit where Athlon has 8. Pentium 4 has 8, and I believe I read somewhere it has 112 other invisible registers. Doubling the registers is definitely a good thing as one can lose clock cycles even when requesting information from cache; and with an 18/20 stage pipeline losing a cycle is a big deal.
Also, we have to move eventually as 32bits can only hold the epoch till 2038 if I remember correctly. There are of course other reasons why a 64bit machine is cool. And a less than 20% performance loss in those tests is a small price to pay, as die size decreases we will make up for those speed issues in increased clock speeds within a year or two.
The benchmarks seem pretty predictable though, and it's true that for most people's needs there is no point in having a 64bit integer unit. But in 5 years there most likely will be, so we might as well start upgrading now.
If possible, I'd love to see a correct benchmark of a Athlon64, since all I have seen so far have been 32bit comparisons. I want to see it's 64bit mode compared to it's 32bit compatibility mode.