He's mainly using it for CPU intensive programs (Music software) that only come in 32-bit versions. Should he go for a 32-bit or 64-bit version of Windows?
that's quite obvious: a 32-bit OS.
Will he get any strange side effects from using a 32-bit OS? Like: A few percent slower computation or access to fewer cores.
yes, a 32-bit OS will not support more than 4GB address space. so substracting the pci space and some other config space will leave him with a few bytes less than 4GB maximum addressable RAM even if more physical RAM is installed. The problem is: by default Windows splits the memory into two chunks: 2GB for applications and 2GB for the kernel. however this can be changed to 3GB for applications and 1GB for the kernel. Read this
for more information. Additionally he will (obviously) not be able to use any 64-bit application, if any special 64-bit version of his favourite software will be released.
and no: he will have full access to all cores and all features of the computer (except for the 64-bit registers, path and operations).
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