64 bit processors are mainly geared towards scientific and engineering circles. Eventually they will start showing up in personal computers, but not until 2002 at the earliest. If the software now written cannot take care of the Pentium 4 and show it in its best light, then you can be sure that the performance of the 64 bit processors will be lacking at first as well. Scientific and engineering recompile their code anyway, so it is not a problem for them.
Intel's Itanium has had problems ramping up to speed. At present, there are few 800 MHz Itanium's available. Most are of a lower speed grade, like 733 MHz and 600 MHz. While touring the SGI facilities in California, I was able to hold an Intanium. The interesting features that I saw were:
1) It has a heatspreader attached and the heatsink was attached to the heatspreader
2) For the SGI computer, it had a translator chip to interact between it and the rest of the computer. Before now, SGI has used MIPS chips, and plans to still continue using them for some time. Currently we have R12K MIPS chips running at 400 MHz, and capable of 2 flop per clock cycle. Since the MTH debacle, that made me a bit leary of using Intanium chips in our SGI Origin machines. Now, this does not relate directly to the PC realm, but it was interesting.
Now on to AMD. Not much is known about Sledgehammer. It is supposedly backwards compatible with the 32 bit processors. In other words, 64 bit is just an add-on to the 32 bit processors AMD now makes. For more info, take a look at the following article:
AMD's 64-bit Gamble
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