Running 32-bit software on 64-bit Windows isn't exactly emulation, memory access (which includes memory virtually mapped to hardware) just goes through an address translation. Because of the way Windows manages virtual memory, 32-bit or 64-bit software doesn't have to worry about keeping track of the exact blocks it gets and therefore all software goes through the same kind of address translation whenever it accesses memory. There should be no measurable performance hit.
If the CPU in your PC in indeed 32-bit only, then there's nothing you can do to put 64-bit Windows on it, but I say go for 64-bit if you can. I doubt the next version of windows will have a 32-bit edition, especially if netbook architecture shifts over to 64-bit.