The game should be highly threaded, it has to run on the PS3, which relies heavily on multiprocessing, so it's somewhat of a prerequisite. As for the 64bit version, the only real reason to compile for 64bit is to use more than 2GB of RAM - and as neither console has anywhere near that small amount, I wouldn't get our hopes up.
This is not true. 64-bit programs have a larger address space. Yes, one of the effects of a larger address space is the ability to use more RAM but you can do other things too, such as use memory-mapped file IO for large files. Memory-mapped file IO allows you to map files to a program's memory space and access them as if they were memory. In a lot of instances, it's significantly faster than using stream-based IO (there are a number of reasons for this but it primary has to do with moving file operations to the kernel space since the kernel handles memory mapping and the fact that with standard stream-based IO, you have to copy the file to memory and then seek through it from start to finish to find the part you want). This means faster load times, especially in a heavily modded game. Unfortunately, with only a 32-bit address space (and this being further limited by the 2gb split), it's not possible to memory map large files in their entirety forcing you to either memory map only parts or rely on slower stream-based IO.
In addition, 64-bit mode doubles the number of general-purpose and SSE registers (from 8 to 16), which allows for the potential of faster calculations.
There is precisely zero reason for it. If it doesn't need to use more than 2GB of RAM... there is absolutely no point.
As to the person saying XP is old... so? It's not like Vista/7 are revolutionary somehow other than improving how user permissions are handled and downgrading the sound system (Only one audio output device at a time, really?). Oh yeah and as a marketing gimmick they never made an XP compatible DX10/DX11. It's just not the same thing. As a bonus XP isn't a resource hog like Vista and to a lesser extent 7. Also, the 64-bit version of XP is not made for the average consumer. This means it's probably not a good idea to use it for your games. It won't hurt anything, but Windows 7 64bit probably far exceeds XP 64bit as an appropriate gaming platform. OS upgrades shouldn't happen just for the sake of OS upgrades (though I know marketing and finances dictate this). For that matter, OS bloat shouldn't increase for the sake of it either.
There are plenty of machines that will be able to run this game that still use XP, and that would suffer performance wise with Vista/7.
It's not possible to make DX10/DX11 compatible with Windows XP without basically gutting Windows XPs graphic subsystem. DirectX 10 drivers operate in user space whereas Windows XP drivers operate in kernel space. DirectX 10 drivers also expect a compositor, something XP doesn't have. Doing such would be a nightmare from a support standpoint; remember how bad Windows Vista was with graphics support when it came out? Now imagine that a patch for Windows XP came out and basically broke all your graphics drivers. (And no, it wouldn't be possible to simply use Windows Vista/7 drivers without gutting more parts of the operating system and then you'd basically have turned Windows XP into Windows Vista anyways.) You could wrap DirectX 10 and translate it's calls to DirectX 9 or OpenGL, like WINE does on Linux, but this would also be a support nightmare because you'll never get 100% compatibility with that kind of setup which would basically force game developers to write three versions of their games (DX9, DX10/XP, and DX10-11/Vista-7).
A 32-bit application can use up to 4GB. Some of that is, of course, taken up by the system (like parts of the OS kernel), but a program that is built "large address aware" has a 4GB limit instead of 2GB.
This is only true if the LAA program is running on a 64-bit operating system (32-bit programs are mapped to a 64-bit address space on x64 Windows which means that they technically get 64tib of user and 64tib of kernel address space but the 32-bit program can't see more than the first 4gb). If it's running on a 32-bit system, it still has a 2gb address limit unless you toggle the /3GB flag (you can't have 4GB because you still need some space for the kernel). But this is not recommended, especially if you have large amounts of video RAM as the kernel half is where that goes.
As I said, some of the 4GB was taken up by the system. I keep forgetting that Windows doesn't properly handle PAE, though. But still, a 32-bit app on a 64-bit OS has the same benefit as with PAE (as in, it can have its own physical memory area that's not completely shared with other 32-bit apps, assuming there's enough on the system).
Windows does properly handle PAE and if you use a 32-bit version of Windows Server, you will be able to use PAE in it's entirety. PAE is hardcoded to be disabled on consumer versions of IA-32 Windows not because Window's PAE support is broken but because there are a lot of poorly written and broken drivers. PAE mode changes the physical address space to 48-bits (although application programs still receive a 32-bit address space). A lot of 32-bit drivers have poorly written, hard-coded assumptions and break when the address space is suddenly changed from what they are expecting. This primarily has to do with drivers that use pointer arithmetic where they don't detect the address space size and adjust the algorithm accordingly. Such as casting (converting) a pointer (a pointer is basically just a memory address) to an integer rather than an INT_PTR. (int is 32-bits on both 32 and 64-bit systems which means on a 64-bit system, the 64-bit pointer will be truncated when it's cast.)
Originally, PAE support was fully functional with Windows XP but they patched that out in Service Pack 1 because it was breaking too many things and causing issues.