the actual max addressable ram is not something that is os dependant its more of a hardware thing
a 32bit cpu can address
4294967296 addresses which is the same as 4gb of ram
and you may think well thats fine because i have 4gb of ram however the hardware can only address 4gb for all addresses and other addresses like pci cards and graphics space is alos adressed in that 4gb allowance
so in effect the systemw ill only address 3.5gb of ram
whereas if you use a 64 bit processor is can address 17million gb of ram
now most hardware sold today is 64bit compatible but you still need to run a 64 bit operating system to address that much