These days, if your system can run a 64-bit OS, you should run a 64-bit OS. There are very very few exceptions to this rule, but they're mainly for backwards-compatibility. 64-bit operating systems can run 32-bit applications just fine.

If you multitask while playing WoW, you may be better off with the 32-bit version. WoW in a 64-bit environment will actively try to use more than 2GB of RAM, meaning your other processes won't have as much available to use. For example, on my Mac I currently have 4GB of RAM, and when WoW is running it uses its 2GB. There's another GB as wired memory, meaning it can't leave RAM, which leaves 1GB for all my other processes. Normally this isn't a problem as I only have iTunes actively running in the background, but if I start trying to run Ventrilo, Safari, Mail, etc it can get strained. If I were running 64-bit WoW that 1GB would shrink quite a bit, probably to near zero.

Luckily for myself I just ordered another 4GB of RAM so I'll be switching to 64-bit the moment it's released (even if there's no Mac version, as I have an Ubuntu partition).

there is a common misconception about the 32-bit wow being stuck at 2 gigs though. That was true up until 3.3.5 but no further, i'm sure some remember always crashing from out of memory errors in 3.x at end of expansion when you got a good enough computer to crank it up to ultra then hit some highly detailed zones or fights

wow 4.0.1 and higher enabled the memory flag for wow to use 3.5 gigs of ram however. I've seen my 32 bit wow use as much as 2.41gigs on just High settings with very minimal mods (lua memory only 8megs), this isn't even ultra.

You also have to realize that 32 bit apps on a 64 bit OS have to load additional resources for compatability. You lose minor performance going through those compatability apis instead of a true native 64 bit app on the 64 bit system. That is where we see the gain really. The extra memory we are gaining with 64 bit client at the moment isn't all that relevent, sine i've yet to see a wow to date reach 3.5 gigs anyways. That will no doubt change in MoP though which is probably what incited this change. Any system running only 4 gigs of physical ram needs to update though cause they have been too low even for 32 bit wow since 4.x. Considering wow can easily use 2.5gigs by itself, and OS uses a sizable chunk add in things like itunes or twitter or aim in background, and you've been hitting Virtual memory for quite some time now. I strongly recommend anyone on 4 gigs of ram to go at least 8. 8 is almost a standard minimum these days for a gamer, people in professional production, even more.

It is good news for (mostly) everyone, but I think it will definitely help out those who run WoW through wine, or similar, on Linux.

The largest difference, is that in a 32-bit operating system using a 32-bit client, the operating system is maxed at 4gb of memory, while your client (WoW) is maxed at 2gb of memory allowable. (Windows does this to ensure that Windows, always has enough to run and your client cant hog all available memory.)

In a 64bit Operating system running a 64-bit Client, if you have more than 4gb of memory, your operating system registers it all. Your client, is also NOT restricted to 2GB, meaning a 16GB memory computer, can use all 16GB.

That's a huge difference, and for memory intensive games like WoW, a huge benefit.

Just gonna post this here for people to see.

http://www.wow-serbia.com/vesti/world-of-warcraft/wow-64-bit-client-test-201201093232/

There's an english translation after the Serbian text, and it might give you an idea as to some of the numbers we're talking about here.