I don't think there will be a patch for XP or 2000 to 64-bit, like there is no patch for Win98 to 'upgrade' to XP or 2000. So you'll have to buy a new 64-bit version of windows. I suspect there will be a cheaper price if you already have XP though, but it won't be free...
Applications which are written for 64-bit will still work though - you don't <i>need</i> a 64 bit OS to make use of it. It's just a 'pure' 64-bit OS would help in a few areas like memory handling and the like. There wouldn't be much performance increase in the OS at all.
There are a few instances where 64 bit registers will speed applications up (where they're written to use them that is), but there is not going to be much gain except in certain specialist software.
In a few years, with games and apps getting bigger and bigger and needing more resources, the need for native 64-bit support will be more apparent, but it's simply not much of an issue at the moment.
There's no reason to not buy an A64 at the moment - I personally wouldn't, but that's 'cause I upgrade frequently anyway so if I was buying <i>now</i> I'd want the best 32-bit performance I can get - as that's all I'd use - but if a PC is going to last a few years then a A64 is probably a good choice, as it's a pretty decent CPU in 32 bit stuff too, and it's a little more future-proof in that respect.
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