You'll have to do a search in the forum. This has been discussed in length before. I don't mind repeating it shortly here.
Hardware: you'll need
1. Notebook (fast with lots of RAM - at least 1GB)
2. MIDI interface - if you're not hooking up to other MIDI gears (probably not especially since you want portability, you won't be bringing lots of other modules), the midiman 1x1 USB interface is excellent with near zero latency.
3. Sound card - the best and easiest to use for notebook will be the Echo Indigo.
Softsynths/ samplers:
1. Garritan Personal Orchestra - with build in Kontakt sample player (so you don't need to buy another softsampler). It includes a Steinway Grand as one of its instruments. This will cover your piano and orchestral instruments. It is only 279USD and includes Cubasis VST and a notation program.
2. FM sounds - Native instruments FM7 or if you want it cheap, their FM7 Xpress is only 49USD.
3. Organ sounds - Native instruments B4, or the cheaper B4 Xpress, also at 49USD.
Most of your bread and butter keyboard sounds are listed above. Should be able to play most songs. If you want pads, Spectrasonics Atmosphere is the one to go for - beware of CPU hog. If you want analog sounds, Arturia's Minimoog or Moog Modular V - again CPU hog.
For the most realistic piano sounds, you will need to buy a softsampler - either Gigastudio or Kontakt. A good investment but not cheap. If you are into heavy sequencing, Gigastudio is recommended. If live, I suggest Kontakt for ease of use. With these you can have:
1. Sampletekk's
Black Grand (Steinway D - 48 samples per note)
2. Sampletekk's
White Grand (Mainly for pop and contemporary playing - 32 samples per note)
3.
PMI Emperor (Bosendorfer 290 SE - also many samples per note: 12 pedal down, 12 pedal up + release samples)
4.
PMI Old Lady (1923 Steinway D - as many samples as the Emperor)
And many more others!
Now you'll only have to bring one keyboard (light controller) with one notebook. Everything you need for sound is in the notebook. Plus the sound is better than any keyboard/ module can give - at least for piano and orchestral sounds. That's portability.
