1. What good is an OS that doesn't detect how much RAM you have? It's not cost-effective to buy 4GB and then use 3.5G of it.
2. I wasn't aware XP64 was out of its beta stage. I haven't seen it available anywhere so I assumed it was still beta.
3. Indeed there has

That's why my first statement was "either he's not running Windows..." - and linux is also not the only 64-bit OS.
Oh, and another thing...
I was digging around my linux kernel and noticed that there were a couple of high memory options... (check under the processor options)
Disabled (I believe it doesn't quite go up to 1 GB)
Support for up to 4 GB (like we're talking about)
Support for up to 64 GB... hmm... not tied to the 64-bit machines either as far as I can see. On the other hand, I doubt it would be very fast as each address would be beyond the native range (if it even worked).