Even though XP supports up to 4Gb, the virtual address space of processes and applications is still limited to 2 GB unless the /3GB switch is used in the Boot.ini file.
So if you go over 2GB, the performance won't really change unless you know how to pick your system.
The LargeAdressAware (LAA) Bit must be set for the .exe file and .dlls, too.
I made a hack for Gothic 3 over at WoG:
Unzip LAA_(Version2).zip to your Gothic 3 Folder and click on LAA_all_ON.cmd.
With LAA_all_OFF.cmd everything will be changed back to original.
Alternative: you can use NicoDE's tool (LAAHack) for Gothic 3 and other games:
For Win32 systems the switches /3GB /USERVA=2800 must be added to the boot.ini
- for Win64 no boot.ini changes are needed.
To manipulate boot.ini you can use this tool -> Boot.ini C.U.: