What types of sounds are you looking for ( general pallette or specifics like guitars , etc) , and what will be your uses ( live play, studio work, home entertainment, etc ) ?
We ask the question here often when someone inquires about about arrangers and synths, but nowhere does it apply more than in the softsynth world, because there are just so many choices.
There are many approches available. You could buy a gm+ wave based module like hypersonic or hypercanvas ( good sounds overall, not the best ), and even improve upon these by opening up in a program like chainer that allows you to chain good effects or layer 2 or more synths ( even the same synths and sounds... for fatter sounds ).
You can go with an app like kontakt, and as Frank points out, get better samples, or if you are looking for specific sounds, you could go in the direction of a dedicated module, such as NI's B4, for example, or DaSample's Electro Acoustic ( excellent guitars ), or a dedicated virtual analog synth like Pro 53 or CS80v.
Ther are so many ways you can go with this stuff. I agree with Frank about the extra hard drive too. In my current setup though, my laptop solely functions as a softsynth in Xlutop chainer, while my PC does all the other work ( recording, sequencing , running apps like OMB, etc ). You could run Kontakt in an app like Chainer ( or FL studio for example ), and chain some external effects with it. There are some high quality effects out there.
When I use the laptop live, then I use OMB, but I try to keep the overall CPU load a little lighter ( less CPU intense softsynths, soundfonts instead of individual synyhs for each instrument, etc ). Since you are looking strictly for sounds, this shouldn't be an issue for you. Still, where you want to play and how is, because some apps are easier and faster to work with than others, and this may limit what you'll want to work with a bit in a live setting.
AJ
[This message has been edited by Bluezplayer (edited 05-14-2005).]
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AJ