Apologies for causing confusion. MS has TWO 64-bit versions of XP - XP for 64-bit is the Itanium version, XP for 64-bit Extended Systems is the AMD64 version. The latter will also support EM64T.
I was addressing the point:
"AMD and INTEL do both have server processors that are TRUE 64-bit, but those are for SERVERS, not normally compatible with desktop OS, and Windows doesn't have a 64-bit desktop OS, "
by saying that AMD don't have a "true" 64-bit CPU in the same way that Intel does (Itanium), the Itanium is compatible with XP (the definitive* desktop OS), and Microsoft does have a "true" 64-bit desktop OS, as well as its AMD64 version (the one that's not out yet).
If you wanted to, you could run 32-bit Windows on an Itanium, but no-one would want to. It's about as fast as a 1.5 GHz P4 in 32-bit x86 stuff, and will only strut it's stuff with programs written for its own architecture.
AFAICT, Intel reverse-engineered AMD64 to produce EM64T. Intel intended to implement all the features of the x86-64 instruction set, including the extra registers the instruction set requires, and the 64-bit memory addressing it enables.
It may be less efficient than AMD's implementation, and the EM64T chips lack other advantages of the AMD64 chips, such as the on-chip memory controller, NUMA & hypertransport for multiprocessor systems. But they should be fully compatible woth XP for 64-bit extended systems and AMD64 games.
*Or at least, most prevalent. I don't want to get into a Linux vs. Windows debate.
Again, I'm trying to balance brevity with completeness, but I hope I've made things clearer.
X2 4200+, XP-90 + Nexus 92mm, VTX 5750, SLK3700-BQE + AcoustiPack, 2 Nexus 120mm, Asrock 939Dual-SATA2, Raptor 74GB, Pioneer DVR-109B, S12 430. [url="http://www.silentpcreview.com/forums/viewtopic.php?t=16171"]SPCR thread[/url].
[url="http://www.silentpcreview.com/forums/viewtopic.php?t=48281"]HTPC build[/url] - NSK2480, Biostar TA690G, 4850e, Asus EAH5570 Silent, Minja, 4 GB RAM, WD GP .75TB + 1.5TB, iHES208 BD, Win7MC.