to Nigie's questions...
Nigie,
The salesguy was a little woofy there. Yes, you can run softsynths and use the MIDI interface with a keyboard to play the softsynths. But this
"He also said these sounds once saved as midi data can be burned directly to CD as wave files."
is misleading. The sounds cannot be saved as MIDI data. MIDI dta is MIDI data. But the recorded MIDI data can certainly play back through the softsynth, just as you could play it through the MIDI keyboard and MIDI interface. And this audio data (produced by the softsynth in response to the MIDI messages) can be mixed down and recorded to CD just like any other audio data.
Now to the General MIDI point - "I asked about general midi and he said that's what you need a sound module like a yamaha (XY?) for. But surely if softsynths can use a standard soundcard to generate synth sounds, there must be some softsynths that can apply a gm format to sounds as an option?"
Yes, in fact, they probably almost all do. So you do not need a hardware MIDI synth for GM compatability, so suggesting so to you was misleading.
Now to the crux of what you're really asking here: "If [what the saleman told you] is true is the soft synth option then a good one to take?"
Well, maybe. The thing is, software synths can require a lot of your computer's resources. They will compete with your recording application for these resources. So as a result, using a softsynth will likely reduce the number of tracks and/or the number of plug-in effects you have applied in your recording application. If your computer is a savage new cutting-edge box and you are not typically pushing it to the max with effects and large track-counts and digital video running along with the audio, etc., then a softsynth might be just the ticket. If you have an older computer it might be a problem. Of course, different softsynths are more-or-less resource-efficient depending on how well they are programmed...
Hope this helps a bit.