I want to build a new PC, and WoW is the only game I play. I simply want it to run with every option maxed out in 1920x1080 on a single display. Can I stick with a 32-bit OS, or do I need to go 64-bit?
(P.S. I don't have any idea what parts to buy, there are too many these days. No actual computer exists yet.)
This is outdated now; please see Ben's answer.
The World of Warcraft client is a 32-bit program. As such, running it in a 64-bit OS provides no benefit except possibly extra available memory. In fact it may be slower on a 64-bit OS; Windows 7's WOW layer, which essentially translates between 32 and 64 bits, is naturally slower than native 32-bit Windows 7.
As for the memory, a 32-bit OS generally only allows for 3-4 GB (see info for Windows). This should be enough for WoW and the rest of the system, depending on what you have running in the background. If you want a lot of other apps using memory, then you should probably go 64-bit with more than 4 GB RAM.
Again, I can't see this being a problem. The hardware requirements for Wow: Cataclysm state that only 2 GB of total memory is required, so having 4 GB in the system should be plenty.
That all said, if you're building a new computer this shouldn't be an issue at all. WoW should run great if you have a good enough graphics card to support the resolution and maxed options. In that case I would go with 64-bit myself, since there will be more and more software that takes advantage of it (and you might want to play crazier games that need more memory than WoW does).
WoW does have a 64-bit client now, so the previous answers are outdated. It will still run on a 32-bit, perhaps even suitably if you don't raid or hang out in crowded cities. However, the recommended settings have been updated also, and 64-bit is now recommended. This is mainly because, as the previous answers mentioned, a 32-bit computer is limited. Blizzard recommends 4 GB of RAM and a powerful graphics card that probably won't work in a 32-bit system.
In any case, you will at least be able to do more multi-tasking on your computer with WoW up if you have a 64-bit system, compared to a 32-bit system with the same specs.
Once this answer is outdated, the entire question will most likely be irrelevant as well, since 32-bit OS's and programs are being phased out.
I would suggest 64bit, even though WoW is a 32 bit executable. WoW will certainly execute on Windows 64bit (I've done it on Vista in the past), but you won't see any direct gain from 64bit since it's not compiled that way. The benefit you will see is from the amount of RAM allowed by 64bit. While it will still be limited to 2GB as a 32bit process, other things on the computer will not be competing for some portion of a maximum 4GB, including your graphics card.
Having 64bit will allow you to have a full 4GB of memory and a powerful (512MB+) graphics card. That should allow WoW to consume all the memory it can, and let you keep music in the background or whatever. Obviously you'll want a multi-core 64bit CPU as well.
The biggest difference you're going to find is in memory addressing.
AWE is used by 32 bit Windows Operating systems. AWE is essentially a 2 step process, PAE is a 3 step process. So architecturally memory access is slower in 64 bit systems (Since Microsoft uses PAE as the memory management standard for X86_64 and AWE was the standard for 32 bit systems (you could enable PAE to get the extra gig of ram on an XP system, but this didn't make you 64 bit.)
64 Bit applications will be able to take advantage of the X86_64 instruction set, and 32 bit ones will require an "emulation layer" (that's not really what it does, but we'll pretend.) Further you will be able to address more of your memory for specific applications, and take advantage of 64 bit operations outside of WoW.
Wow utilizes DirectX like most other games out there. DirectX (unless built using the 64 bit SDK) has a limit on object sizes of 2gb, so there's no way you'll be getting any more memory use out of the system than you were before. And since 64 bit addressing is slower, it's not a performance gain there.
Honestly, if you want a performance boost the biggest bang for your buck would be a solid state disk that you have WoW loaded on. That and your GPU are the two system components that you're turning to slag. The memory component is really only an issue if you're constrained and paging to disk.
In terms of which to buy, I'd go with 64 bit since you're not really going to get a long life out of your 32 bit OS whatever it may be. Hardware vendors are going to eventually stop writing driver updates for your 32 bit OS and 64 bit is what the software companies are starting to focus on. Eventually It's likely there will be a World of Warcraft 64 bit client, but Blizzard will have to find a use case for storing more than 2gb in memory at any one point.
The bottleneck of the WoW program is the GPU. Running it on a 64-bits or 32-bits OS won't change it, making no difference on the FPS of the game.
It will run as a 32 bit process within Win64.
That introduces some overhead which on low end machines might reduce performance a bit.
But on higher spec machines the extra memory 64 bit Windows can address helps, as there'll be less swapping going on within other processes running concurrently, thus less expensive disk access and as a result a more fluent experience.
So you can't say it will definitely help or not, just that it might.
