Better Software or Better Soundcard

I'm new to the world of MIDI and am trying to use the quality action in my digital piano to play and record tracks using software on my PC. I've set up a laptop and purchased a Midisport 2x2 USB Midi interface to connect my laptop to my digital piano. I have downloaded half a dozen demo MIDI sequencer applications like Cakewalk and FL Studio which have great features but fall short on producing the properly with the action of my DP. (sounds fall off too short, are delayed (samples), and in some cases cannot be played on more than one key at a time)

My goal is through a combination of hardware and software to in real time assign instruments and instrument layers to keys to allow live playing with many voices. If I understand MIDI correctly my digital piano should send data on the key played, velocity, duration, and even pedal action, and then my computer should be able to assign one or many sampled or synth sounds to that data in real time.

I researched many forums and found terms like, SoundFonts, VST, etc... that seem to suggest that some sounds are sampled others are synthesized and many require a sound card.

Currently I have a laptop running XP at 800MHz that I can run my software on. So I am wondering what software can I use to play QUALITY sampled or synth instruments and if I need a sound card what would work for a laptop.

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