if you want something new, something like the X50 would be great.
cheapest option for a piano sound would be to pick up an old rompler rack
unit, like an old roland jv series or somethin
or see if you see a second hand triton rack goin cheap.
or there are many keyboards new or old that would do the trick.
but to simplify everything for you
there's keyboards that use synthetically generated wave forms as its source material, and usually filters this signal, may modulate its frequency or do other weird and wonderful things to it, and may run it though fx, and supply the user with various modulation options to get a sound.
these are great for getting specific types of sounds that dont really sound like acoustic instruments at all, though many classic synth sounds are from people attempting to recreate 'Real' sounds using this archetecture and without samples.
then you get keyboards that are based around samples, often stored to Read Only Memory (where the word ROM comes from)
samples are edited recordings of real instruments
someone presumably sat down with a piano and sampled a few keys, and transposed these samples over the rest of the keyboard. this is sometimes called a multisample.
the keyboard comes with a selection of these multisamples
this multisample is often layered, run through filters and various fx etc etc
and often has various modulation options
but most do not have sampling capabilities for the user to make your own samples. these are nicknamed ROMplers
some keyboards have their own sampling capabilities, letting you record or import your own samples, most keyboards dont have this capability.
this sample is often layered, run through filters and various fx
and also often has various modulation options
these are samplers
you get keyboards that try to combine multiple things
like sampling, Rompler attributes and maybe a sequencer
these are often marketed as workstations
for pianos and "real" sounds, piano's its best to get a Rompler keyboard
for best synthetic sounds... get a synth
i dont know who sold you a microkorg telling you it could do realistic piano
with a simple 2osc synth engine, (which does include some more complex synth waveforms...which can get some keyboard like sounds)
no samples and 4 notes of polyphony..
there's a lesson here in doing research before buying gear.
heres what you want
http://en.wikipedia.org/wiki/Rompler
get this
be happy