A few questions first:
1)Why do you want to switch from 32-bit to 64-bit?
Most people tend to switch because 64-bit OS do not have the 4GB Memory or RAM limitation that 32-bit OS have. So that way they can get as much RAM as they wish (8GB, 16GB or higher) and not be limited by how much Windows will limit or take advantage of. Additionally, 32-Bit Windows versions (both XP and Vista) don't really give you all 4GB of RAM even when you do have it, as about half of it is reserved for windows processes; so technically even with 4GB RAM you only have around 2GB or so available for your programs to use. But there's a work-around to this; more on that in a bit.
However, all this, (switching from 32Bit to 64Bit, and getting more RAM) only matters if the actual programs you plan on using will take advantage of more than 4GB of RAM, i.e are 64-Bit programs. 32-Bit programs cannot see more than 4GB (or what''s left over from Windows) of memory available, and so don't ever take advantage of it. So even using a 32Bit program in a 64bit OS is kind of pointless from the perspective of having more than 4GB of memory since the program will not take advantage of anything above the 4GB.
2) In the event that you don't plan on getting more than 4G of memory, have you considered getting a 4Gig switch for 32bit Windows?
Basically this will save you the trouble of re-installing OS's, and allow your programs to take advantage of pretty much all the memory you have even in a 32Bit environment. It's a component that circumvents 32Bit Windows' 2Gig memory limitation, and releases most of it to the programs available.
The reason I ask these questions has also to do with the fact that ArchiCAD (as of right now anyway - and which I assume is one of the reasons for a switch), is still a 32Bit application, and so would never gain you any advantage in switching from an 32Bit to 64Bit OS. In other words, it's not going to use more than 4GB of memory even in a 64Bit OS environment, and so if this is a primary reason for a switch, then as far as AC is concerned, it's kind of pointless, atm.
You might have noticed people have been begging Graphisoft to migrate ArchiCAD to a 64 Bit platform or at least allow for both options and frankly speaking, for a mult-core aware application like AC12, it's kind of retarded that it's still just a 32Bit app, even when Microsoft themselves have stated that they will be abandoning the 32Bit platform, and 32bit Windows in a couple of years; and more importantly even when most of their users are already migrating to 64Bit Windows, and getting more memory for their applications that are 64Bit - (including non-multicore aware competitor Revit, which is available in both 32bit and 64bit
versions).
Further to which, there are also other advantages of using a 64bit application in a 64Bit OS, beyond just the memory since there generally more resources available to the programs, and they're not constantly fighting with Windows for stability - i.e less crashes, faster. I'm currently using 64Bit Win7, and my 64bit applications just fly and are way more stable than the 32bit versions also in the same system.
Who knows, maybe we'll get lucky with AC13.
I should warn you though, that 64Bit Windows (XP, Vista and Win7), along with switching to them, do tend to have problems - primarily to do with hardware, and drivers not supported. So here is where there may be issues in switching - be sure to check that your components have 64Bit capable drivers first, otherwise the switch may be more of a headache than a benefit.
But in terms of what you'll have to go through, you'll inevitably have to re-install all your programs, since, the directory structure for 64Bit windows is different to the 32Bit version, and I don't really think you have to pay any upgrade fee for any of your programs unless you plan on getting 64Bit versions (and assuming that they don't already offer the option when installing to install as a 64-bit application - most programs will automatically detect a 64-bit system and prompt you to install the 64-bit version; if available).
Oh, and stay away from Vista - far faaar away- especially, if you have the option. Even though 64bit Vista is still more stable than the 32Bit version, I would still advise going with either 64bit Windows XP or preferably 64Bit Windows 7 (my choice). Vista is the reason Microsoft are so anxious to release Windows 7 as soon as they most possibly can.
Again, I repeat, 64Bit OS is NOT going to gain you any advantage (memory-wise, resource-wise) from ArchiCAD that you wouldn't already have with using ArchiCAD in a similar 32bit OS with a 4gig switch. Other programs (64Bit ones) may, but definitely not AC. So weigh that against any potential decision to upgrade and re-install everything all over again, along with all the hassle that comes along with it. ArchiCAD is not a 64-bit application, and unfortunately them's the breaks, until GS decides otherwise.
