The new ARIA player for the Steinway has an x64 version that works in Windows XP x64. Gary mentioned in the Updates to the Updates that they plan on porting their libraries over to ARIA player in the future. I'm sure he will want to get the General MIDI library out first before updating the older products though. So there is a good chance you will see a x64 version in the future for GPO.
As far as I know KP2 already works in WinXp x64 as a x32 application, which you can get to load at least 3GB (you may have to use the 3 GB switch). Being limited to 3 GB shouldn't be a problem with GPO - it's not a big library.
The x64 version will basically be the same version as the 32 bit. It won't change the sample rate. GPO currently uses 16 bit samples to save on memory size. What the x64 version will allow is more memory to load samples especially if you mix different libraries together that a x64 compatible. This comes in really handy if you GPO along with Vienna (VSL) libraries for example.