It does run under x64 but you are still limited to 2 GB. Since Kontakt is a 32 bit app, it has to run thru BitBridge in Sonar x64. BitBridge is not a perfect solution and some plugins have issues remembering settings when you reload the song.
Another option is to run using the standard 32 bit version of Windows XP with 4 GB of memory in the machine. Then add the /3GB switch in the boot.ini file. Then run a program called Laatido to make Sonar LAA (Large Address Aware). This will then allow you load about 2.7 GB of samples. Sonar 7 is already LAA so you don't need to run this program on it. This is how I'm running my system at the present as I have a few plugins that don't work properly through BitBridge.
It looks like option 2 may be the easiest route to take for the near term. I'm building a new box w/ 4 Gb RAM so if and when all my music apps will run under XP64 without bit translators, I'll be set. I'm upgrading to Sonar 7, and thanks to your helpful info, at least I'll be able to increase my current 2 Gb RAM limit by a 1/3.
I have both Windows XP 32 bit and an x64 partitions. That way I can test both OS's. Gary's new sample engine will have both 32 and 64 bit versions. It appears that a large amount of developers are coming out with 64 bit versions in the next half year. Unfortunately, Native Instruments doesn't appear to be going 64 bit at all.