I went to fire up Windows 2000 in a VM this past week. I had completely forgotten it was the last 32-bit version. If it were 64-bit, I expect there would be many still in service today.
There isn't one for AMD64/"x64", only for Itanium and DEC Alpha, and the Alpha version was never truly released so there's only an RC2 build for that platform.
While there was a 64-bit edition of Windows XP (actually two: one for Itanium and one for AMD64), it was not very popular or well supported by drivers.
It eventually did get nearly full driver support, the last pain point i had on WinXP x64 edition was ASIO4ALL, for years the author said, paraphrased, "I will never support 64 bit OS" - although it works fine on win7 x64 and up.
The drivers that never worked correctly were industrial and "weird" things like radio programmers, stereoscopic headsets, and limited run PCI[e] cards and specialized USB peripherals, but most of those had issues with XP 32-bit as well, and never worked past XP, in general.
