In regards to Win 7, in 32bit mode, it caps ram at 4GB (3.25GB usable). With 64 Bit mode:
Here are the upper RAM limits for the different editions of Windows 7:
- Starter: 8GB
- Home Basic: 8GB
- Home Premium: 16GB
- Professional: 192GB
- Enterprise: 192GB
- Ultimate: 192GB
Source: http://www.zdnet.com/blog/hardware/max-memory-limits-for-64-bit-windows-7/4254