My PC came with 7- 64 . I dont like it much because many programs are available for 32-bit and not 64-bit. But that seems to be changing - i saw 64 bit versions of eclipse, tomcat, etc.
PS : The windows 7 has this wierd UAC , it puts irritating restrictions on installation and execution of software but not on malware, one of which had bypassed UAC and attacked silently. IMHO, you should not upgrade if all you do is "basic work".
More memory would be my main reason for shifting to 64 bit Windows. I am using Windows Vista on my main machine right now, but my wife's new PC is running 64 bit Windows 7, which looks pretty good to me so far. No problems with UAC (unlike Vista), and her 32-bit programs seem to work fine.
If you want to use more than 3.2 GB of RAM (varies a bit) then you have to use a 64 bit operating system. Modern high performance systems use 6GB or 8GB or more. My most recent laptop has 8GB, as does my 18 month old desktop. You don't have any choice, 32 bits can only address 4GB of anything. (There are weird hacks like PAE, but they are not general solutions).
If you have an old PC, you may not even be able to install more than 4GB, so for that, 64 bit code has no advantages. It is likely that using 64 bit code takes a little bit more RAM for all use, a 32 bit address can become a 64 bit address, but this is usually not a big deal. The big deal is using lots and lots of memory.
All of this is true for Windows and Linux. Apple's Mac OS-X has been all 64 bit for years.
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Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop