STAM, Kontakt is one of the top of the line soft synths. I have it and it is very good indeed. The current version 2 has some bugs but it is still very good. These bugs will be fixed by the 3Q of this year according to Native Instruments.
Kontakt, comes with many sounds/instruments. Some of them are very good. If these are not good enough for your use you can always go and purchase new sounds which are the best in the business, e.g., Garritan Strings, Bardstown Bosendorfer Piano, and so on. Here are some peoples views on the NI K2 Library:
http://www.nativeinstruments.de/site/forum_us/showthread.php?t=25137 For this to work you would need a fairly good quality computer/laptop, soundcard and a second HD with 8mb Cache and 7200rpm. This is necesary since the instruments are so large they are streamed from the HD as you hit each note. Only a small portion of each sample is loaded into memory.
Furthermore, the way I setup my lead/melody voices, I load separate instruments in each of 1 to 8 (your could go to 16). I then switch channels on my controller keyboard to play the instruments I need. You can, of course, layer voices as well. You can load up a different set of 8 (or more) instruments as often as you need to....a different set for each genre of music, etc. Loading 8 very large instruments takes about 30 seconds. If you use very small instruments like you would find in your Korg or any other arranger keyboard it would take less than a second.
Other soft synths or sample players of this kind include Halion, GigsStudio, etc. In addition there are other soft synths which model acoustic or synth sounds, e.g., NI B4 models B3, etc.
I hope this helps.
[This message has been edited by Frank L. Rosenthal (edited 05-14-2005).]