I have the Diamond collection from KHS which IMHO is fantastic (I also got them on a group buy, which made it quite easy). Right after installing the download I played a few chords, and just said WOW. The quality was/is simply remarkable. I also have Miroslav's full version and GPO, but had to "massage" them a bit to get the desired sound. The latter two are fairly good, but the KHS just outshined them.
If you're looking for something straight out of the box, with very good quality of sounds, I highly recommend KHS (your system seems more than adequate to run this smoothly). I have an older system which hasn't given me any issues, but then again I've only used 1 instance of Kontakt with a few ochestral libraries loaded at one time.
Best of luck!!