Hey,
If you read electronic magazines, they often talk about this subject as a full blown orchestra is a pretty cool thing to have in your music/soundtrack, and there is an art to replicating it using synths and samplers.
From what I remember of the articles, there are a few things to do:
1. Lots and lots of different instruments: strings (violins, violas, cellos, contra-basses), brass (english horn, french horn, baritone, trumpet, trombone), woodwinds (clarinet, bassoon, oboe, flute, piccolo), percussion (timpanis, cymbals).
2. Lots and lots of layering. A real orchestra will have tons of individual players all playing the same thing or variations of the same thing. There is usually some sort of 'theme' that most players follow, the rest will play harmonies or other melodies which compliment the theme.
3. Dynamics. Something you don't hear very much of in pop music. LOUD passages, and quiet passages.
4. Good sequencing. You'd probably want to use a software sequencer to make a score-type track, trying to accomplish this with something like an MPC could be very tedious as you only have a small screen on which to edit. Things like modulation and volume ramps are much easier with a mouse.
5. Tuning. If you have two violins playing the same melody, it will have a 'thicker' sound because they aren't 100% in tune with each other. You can fake this by running sounds through a light chorus effect, it will give the impression that there are more instruments playing.
6. Reverb. Concert halls have a certain sound about them, big and roomy. If you put a dry string sound (or the whole mix) through a reverb, it will give the impression that the music was performed in a hall instead of a dead room.
7. Panning. If you look at a real orchestra, you will notice that like instruments are placed in groups across the stage: the flutes front-left, clarinets front-right, low brass high-center, high brass mid-center, percussion top left and right...imagine your sequenced orchestra like this, pan your instruments accordingly.
Take care,
Nick