64位体系结构的32位操作系统

I am running 32bit ubuntu on 64bit x86 processor (intel). I know that the word size is 64bits in this case but I am little confused about the 32bit OS. So while I calculate the memory bandwidth, shall I assume that the data bus width of 64 lines will be used and it will exhibit the same performance as the 64bit OS? IOW, I want to better understand the relation between OS width on the architecture width.I am running 32bit ubuntu on 64bit x86 processo