TerraTec 512i better audio-card than SoundBlaster Live???

[Teemu]

I wonder, what's the difference between "soundblaster compatible MIDI interface" (which this card has) and the "General MIDI" of my keyboard?

I guess "soundblaster compatible MIDI interface" means that the connector is in the PC gameport format, as opposed to the standard MIDI connector format.

[Teemu]

Do one need some adapter to use the game-port for this(apart from the cable keyboard <-> soundcard having fitting ends)?

You'll need an adapter cable with a 15-pin plug on one end to fit into the gameport and two standard MIDI plugs on the other to fit into the keyboard's in & out connectors. I haven't looked myself, but I've heard these sort of cables should be widely available from computer and music stores.