I have somewhat older 64-bit machine that I need to install Windows XP on. I will need to purchase Windows XP Pro but I'm not seeing anything about a 32-bit or 64-bit version. Does it matter or can I just buy Pro and it knows what to install when it detects my processor?
The short and quick: If no x64 is stated on the box, then it is 32bit.
Either will work fine 32 or 64. If you have over 4GB of RAM only 3GB or so will be available in Windows XP 32-bit due to kernel issues. But with older systems this is usually not an issue as 4GB was not the norm until recently.
X64 XP compatible software is a bit of a tricky find so as stated 32bit is probably your best bet.
IF this computer is going to be used for computational programs or something that will utilize the 64bit threads, then I would look deeper into the x64 operating system and if need be get a copy of Windows 7 x64 simply for future support. 2014 is it for XP 32 and 64.
It doesn't matter. I wouldn't suggest the 64-bit XP just due to hearing a lot about compatibility issues with the 64-bit XP. If you install a 32-bit XP Pro on a 64-bit machine, it will be fine.