Give me one please

Ok I know this is off-topic but I think the people in this thread should be able to help. I have an old HP server with four 2.8 GHz Xeon CPUs which are 32-bit. The machine came with 6 GB of RAM in it, and has capacity for 16 GB (but ECC Registered DDR200 is expensive so I won't be upgrading). Which version of Windows can access 6 GB of RAM AND work with these CPUs? I know it'll have to be a server OS as XP/Vista/7 only support up to two CPUs, and I know it'll have to be 32-bit...