To enable 3GB usage, under XP:
- Edit your boot.ini file (hidden, in the root folder) and add the /3GB switch to the startup parameters for XP
- Under Vista:
1. Right-click Command Prompt in the Accessories program group of the Start menu. Click Run as Administrator.
2. At the command prompt, enter "bcdedit /set IncreaseUserVa 3072"
3. Restart the computer.
Note that 3072 it's just a starting value, but if your video card has lot of VRAM, it's better to use lower values, like 2560 for example.