Actually....
32 bits operating systems are capable of addressing up to 4GB of RAM.
However, under Windows 32-bit operating systems you're actually limited to 3GB usable - even when you install more.
http://msdn.microsoft.com/en-us/library ... 78(v=vs.85).aspx#physical_memory_limits_windows_xp
In short, there are differences between what various hardware and software will support and just because the hardware will support it, doesn't mean the OS will and vice versa.

Keep in mind this is mainly a Windows thing....