Apparently (again, just my understanding), the 64 bit version of XP will load the kernel above 4GB when the Opteron is in compatibility or full 64 bit mode. It may well be that an application will have to be recoded to see the extra 2GB, though they don't say that. As for the advantages, large databases and data intensive scientific applications can take advantage of all the memory they can get, one reason why DB2 and MySQL are some of the first "general purpose" applications being ported to 64 bits. The more data you can keep in main memory, the better. Apparently, game makers see a plus also. Epic Games, is doing a 64 bit version of Unreal Tournament 2003 and is working on an entirely new 64 bit game as well. I guess people who don't mind spending $500 for a video card won't balk at the price of 4 GB of registered ECC RAM either.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken