Thanks for the information!
I have a maudio audiophile 24/96 isn't that enough to connect (multiple) 
instruments?
This card (like most cards) has only one Midi I/O so if you want to connect more instruments you need some kind of midimerger like a Midisport, that is when you're using Din cables. With USB everything is much simpler. Unfortunately the NP 30 does not have USB.
About the sounds, what are sounds which are used a lot in 'light' music?
I know and like the hammond organ sounds and the Rhodes sounds.....

Is there a piano / keyboard which can make those sounds too?
I'm in no way an expert on this but I do know that almost every keyboard has quite a lot of this sort of sounds onboard. Yamaha's keyboards, even the small ones, usually have 7 or 8 different piano's and about the same amount of e-piano's like Rhodes sounds and also a variety of organ sounds including Hammond.
Mind you, the Yamaha NP 30 has a lot less: 2 grand-piano's, 2 e-piano's, 2 churchorgans (no Hammond!), strings, 2 harpsichords and vibraphone. Most digital piano's have only this smaller amount of voices, because they focus more on action than on hundreds of sounds.
And how does software sounds on linux (inclusive commercial) compare to 
sounds (modules) of keyboards?
Can you compare the rhodes soundfont with such a sound made with a (not 
to expensive) keyboard?
I've only once fiddled around a bit with a Rhodes soundfont (jRhodes3) and I liked it but I hardly ever use it. To me it sounded quite like the sounds on my keyboards (Yamaha PSR 540 and Roland EXS 3s) but I don't feel myself qualified to really compare these. I did find the dynamic range of the build-in e-piano's far greater than the soundfont's one; maybe other soundfonts are better in this way.

Regards,
Bert