To answer your question second question.I have heard unconfirmed reports that the latency in a Mac laptop is good enough to play software synth without a soundcard. I use an averatec laptop and the echo indego PCMIA card.This card has 24 bit sound and even capability to record two tracks if need be.
As to your first question,I think thats one of the beauties of software synths is that there are many ways you could achieve the same result depending on what you want to do,I heard this corny joke the other day,apparently this guy had a computer as his true love because it could be anything he wanted..........the truth is there,your computer can be just about) about anything you need it to be.
Armed with a computer,a controller,and say a prophet 5 soft synth(there is one)you could arguably do more to the sound than the origional could.
I was looking at that new Korg 8000 dollar monster out there,the KAOS.It is a really nice keyboard,but you can do most all of that with just a laptop and some good software.