In general the ram you can address when using a 32 bit version of any operating system is the amount of 4 GB minus the memory of every other pci device, because that memory will be mapped before using any more ram.
In this case: 6 GB reduces to 4 GB because of 32 bit addressing, and maybe 4 GB - 1 GB because of your graphics adapter memory, and some other stuff you get a total of 2 GB usable ram, the rest is needed for adressing the mentioned graphics memory etc.