Enlighten me, geeks!

[Racing Rice]

That is correct.. You need a fully compatible 64-bit system to run that much memory.

Actually 32-bit supports 4GB and 64-bit Windows XP OS supports 128GB of physical RAM.

Here are some other memory support stats:
64-bit vs. 32-bit architecture
Address space 64-bit Windows 32-bit Windows

Virtual Memory: 64 Bit - 16TB / 32 bit - 4GB
Paging file: 64 bit - 512 TB / 32 bit - 16 TB
Paged pool: 64 bit - 128 GB / 32 bit - 470 MB
Non-paged pool: 64 bit - 128 GB / 32 bit - 256 MB
System cache: 64 bit - 1 TB / 32 bit - 1 GB

So obviously, if you run memory intensive apps you will see the greatest benefit with a 64 bit system.