If his motherboard is so far in legacy status, it might present some issues. I remember going through a similar situation with AGP, when 8x was introduced and different voltages were set, which made near impossible to install an AGP 8x on 2x/4x motherboards without flashing the BIOS.
Now if 2 to 4GB of memory makes a difference, well it does. The whole system is more responsive, and from the moment many games only enable higher settings when running on x64 OSes, that by themselves require more memory than x86, then I'd say memory is important and fundamental.
And OP has not even talked about his plans about OSes, how he plans on getting them, etc.