If you have a 64-Bit Operating system, using the 64-bit version of WoW will allow the game to use more then the default 2gb of ram it already uses. Unless you're a multi-tasker and like to do things while WoW is running, using the 64-bit version would be a Boon.

And if your computer uses DDR3 ram, its pretty cheap at the moment, you can get 8 gig bundles (2 4gb sticks or 4 2gb sticks) starting from 50 bucks...

Yeah.

The 64-bit instructions take longer for your processor to read and decode than the 32-bit ones, so it would actually slow you down to try and run a 64 bit version if you didn't have the extra RAM to offset it.

Conflicting replies are conflicting :(

It's probably OK to go to 64-bit with 4GB RAM. It's a small download and you can choose which one to load from the launcher, so why not try it when it comes out and see what works better?

I haven't done the research regarding Edymnion's assertion that CPUs take longer to perform 64-bit instructions, but I wouldn't think there's a significant difference between processing time for 64-bit vs 32-bit arithmetic when you have a 64-bit CPU. There's certainly a difference when doing 64-bit arithmetic on a 32-bit CPU, because adding two 64-bit numbers on a 32-bit CPU requires two 32-bit additions plus a third addition to carry the one. This probably isn't relevant to WoW anyway, because the 64-bit WoW client will likely still only be performing 32-bit arithmetic to maintain compatibility with the 32-bit clients.

The big difference between the 32-bit and 64-bit WoW clients will be the RAM usage. 32-bit addressing allow you to have 4GB worth of addresses but only half of that is used to access RAM and the other half of the address space is for direct access to hardware, so a 32-bit application can't occupy more than 2GB of RAM because it wouldn't be able to remember where it was putting things after that. 64-bit addressing squares the size of the address space to 16 Exabytes (Exa- is the prefix for 10^18), and even if the tradition of only using half the address space for RAM persists, 8EB is somewhat bigger than we're likely to reach in terms of home computer RAM in the next few years.

The 64-bit client will be able to expand to use all available RAM in your computer and will likely give some performance improvements due to not having to unload and reload big chunks of the game to stay under 2GB, but this comes at the cost of needing to use twice as much memory to remember the location of each item it stores, so it will occupy more RAM to have the same data in memory. To borrow somebody else's library analogy from earlier in the comments, the new index cards that let you access all the extra shelf space are twice the size of the old cards and thus your index card cabinet needs to be twice as big.

All that said, the extra memory required by the 64-bit client isn't going to take THAT big a bite out of the extra 2GB that's available to a 64-bit app on your 4GB system, so you'll still be less likely to run into out of memory errors.

Give it a try. It wouldn't hurt.

I disagree with the assertion that the larger client size (which should be minimal) will make a big difference. The larger amount of memory you can access should more than make up for it.

"The 64-bit instructions take longer for your processor to read and decode than the 32-bit ones"

This is false. All modern CPUs have native support for 64 bits and do not need any special decoding or extra steps for 64 bit instructions. The registers support 64 bits natively. They don't need to use SIMD unless it's 128 or 256 bits (and if you don't know what SIMD is, you probably shouldn't be making these assertions). Unless you're running a CPU older than the Pentium 4, this is simply not the case.

Not only does it not take longer to decode a 64 bit instruction than a 32 bit instruction, it takes the same amount of time to read them from memory. 64 bit processors load up 64 bit registers by reading out 64 bit values at a time. A 32 bit instruction is just going to go through the WOW64 layer on Windows (which allows 32 bit instructions to be executed on a 64 bit processor), which means that it actually takes *longer* to execute a 32 bit program than the same 64 bit program on a 64 bit version of Windows (I can only assume Linux and MacOS do something simliar from an translation layer standpoint).

See: http://en.wikipedia.org/wiki/WoW64