A software sampler(actually sample player) loads libraries by the manufacturer or other 3rd party manufacturers. it does no generates sounds in of itself. With a soft sampler. youu sonic pallette is virtually infinite.
A soft synth usually concentrates on a specific instrument type.
examples:
Native instruments' Pro53

is a recreation of the old Prophet 5

Native instruments B4

well, thats obvious.......
Those are soft synths.
a soft sampler loads whatever you can find.
example:
Native instruments
Kontakt---very good

or IK Multimedia Sampletank

what you want is a soft sampler
as far as the hardware, yeah you do need a sound card. the sound card that's bulit in to the laptop is ok, but there will be a delay between when you hit the keys and when you hear the sound. That's called latency.
A separate sound card lets you compensate for that. doesnt have to be nuthing crazy 99-199$$ is good.
hope that helps.