Hi Pedro
Don’t try too many things at the same time
With Kontakt use the setup feature to load all the sounds you need for a song or performance and then save the setup (Make sure you have Disk Streaming activated)
Do the same for your other songs/performance
If you follow this rule you will only have load times between songs/performance and Not in real time play (Unfortunately trial versions don’t normally allow you to save setups or allow disk streaming)
The accepted standard for pianos is Ivory http://www.synthogy.com/index.html however it does need a good CPU, Ram and Hard Disk (However it is what a lot of professional studios use)
Also try True Pianos http://www.truepianos.com which uses sound modelling rather them massive samples
The best resource I have found for info on VSTs is http://www.kvraudio.com and if you have a look round the site you should find all the fixes you need to get your system running smoothly
Alternatively (If you don’t mind spending) go for one oft software arrangers like Wersi http://www.wersi.net or Lionstracs http://www.lionstracs.com that look and operate like a hardware arranger
In addition as well as OMB have a look at LiveStyler http://www.live-styler.de/home

Hope this helps

Bill