Check if 16GB RAM is recognized by your server with Win2003 in ‘properties’. if it does not, check with server specs how much maximum RAM it can support.
If it supports 16GB and the performance is ditto same as was with 4GB, take a backup of your database, reinstall OS and Oracle 9i and restore your backup. It will solve your issue.
Windows 2003 Enterprise Edition will support 16 gigs of RAM without a problem. Windows 2003 Datacenter edition is not required until you want to use over 64 Gigs of RAM.