Hi veggiop,
If you want to address the full 6GB of RAM you will have to install a 64bit (x64) OS.
Also running XP without any Service Packs is a bad idea as your security will be at risk.
Regardless of which Service Pack you do or don't have installed, you will still be limited by the fact that you are running an operating system that uses a 32bit architecture. A 32bit OS will not be able to address more than 4GB of RAM (in real world probably about 3.75GB)
If you want to address the full 6GB of RAM you will have to install a 64bit (x64) OS.
Also running XP without any Service Packs is a bad idea as your security will be at risk.