In a windows 32 bit environment one process can normally access 2G of ram, regardless of it’s being Oracle or other software. Also the Oracle windows implementation uses threads, taht is the whole DB instance is one process and thus can use normally 2G ram. However windows server can use a /3G switch in the boot.ini file to increase the process space up to 3G. See metalink 1036312.6
This is still far from your total 6G.
Another feature in windows is to use VLM (Very Large Memory) which let’s you use more ram if the program supports it. Oracle does support this feature. For more info look for the initialization parameter USE_INDIRECT_DATA_BUFFERS. See metalink 1036312.6