Would it really improve performance or would it be just a small boost?
If you’re running a 64-bit version of Windows, you’ll see a nice boost because ZBrush could be given more than 2 GB of RAM.
If you’re running a 32-bit version of Windows, the performance boost will be less. ZBrush won’t get more than 2 GB of RAM, which means that any extra will be used for the OS and background apps.