The /3GB switch does not allow you to use more than 4GB of RAM; it allocates more of the RAM to applications than would be allocated by default:
technet.microsoft.com/.../bb124810(v=exchg.65).aspx
You can set the /pae switch, which will allow a 32-bit OS to use more than 4GB of RAM, but there are limitations (not like addressing with a true 64-bit OS), and is ONLY available in Enterprise and Datacenter editions of Server 2003