I've just started working with a singer/ songwriter who's making his first album.  We decided that we wanted to have some strings on a couple of the songs.  So, the first thing I did.... I opened up Dimension Pro, fired up some of the orchestral sounds. 
 
"Batman Score" was one of the coolest orchestral sounds.... so, I recorded a track with that sound.... just playing PADS with the chord progression of the song.  Then, I began thinking...... first of all, I know that there are professional string arrangers that have been doing this for years.  I'm not one of these people. 
 
BUT.... instead of using one "orchestral sound," should I be breaking up my string part into multiple tracks and layering the string parts by INSTRUMENT?  For example, violins on one track, violas on another track, basses on another, cellos, maybe even clarinets, flutes on their own tracks.... etc. 
 
I'd like to have some counter melodies going on, too, so that's why I'm leaning toward going in this direction.  Any thoughts?

Yep, separate tracks for the different instruments is definitely the way to go
You will need to insert as many instances of Dimension Pro as you have instruments, and with 6Gb of RAM you should be good to insert a fair few before you run into system limitations.
Keep an eye on your RAM usage - if it starts to creep up ominously, freeze anything you're happy with. Freeze the synth, not the track.

Unless I need some sort of (almost hidden) 'stringpad' in the background of a song, I always create tracks for every instrument/section: not only so I can pan them as they should be panned but also so I can for instance let one section play crescendo while another may stay at the same volume or play decrescendo. In order to create convincing arrangements, you really need to do this. Sometimes you want a cello to play the melody and sometimes the violins, etc. etc. But even when I let all parts play at around the same volume (as in a string pad) I would create separate 'volume envelopes' (usually done with modulation when you use 'real' string libraries) for every part. It just sounds more real that way. Every section has and needs their own 'flow' and build up etc. 
 
Since you talk about counter melodies, I would definitely go for separate tracks!

depends on the need.  At times the pad is more than sufficient for the "strings",
 
and if the song warrants it... something like a Celine Dion piano based song, yes, you would want to use multiple tracks and plenty of good samples when you want it to sound like there is a real string session back there.

Hey, Jimmy,
     I agree with the other posts so far but there are some other orchestration techniques to take into account. Facing the orchestra, the 1st violins are panned to the right, little off center. 2nd Violins are farther to the right. Violas are are panned a little to the left off center. Cellos are farther left. Bass sections are sometimes behind the cellos and sometimes in the center.
     There is also the split table technique, usually for chord work. This means in order to play a chord, if you have two players at the same music stand, the music would have two notes on the manuscript. One violinist would play one note and the other player, well, you get the picture I'm sure. This is used in violins and violas mostly.
     Real string players can not play full chords. They have to break the chords into separate notes split between the section to get a pad-type chord.
These two techniques can make for some realistic sounding orchestrations. If you use this and do the hard work now, you can just print out your section string parts for real players later.
     I would suggest visiting some websites about orchestrating. Garritan has a free tutorial taken from RIMSKY-KORSAKOV. They also have a great jazz tutorial class as well that is free of charge. Good luck my friend! 

All advice above is good.  The first question is if you want realism or just a cool effect.  For realism, here are some basic thoughts:
 
• Agree to put instruments on separate tracks.  You can Google to get an orchestra seating chart to see where instruments are placed in a traditional symphony, although sometimes in modern music, things are panned a litte more extremely and the basses are kept more central.
• Phrasing is very important.  Try to sing along with a part and notice where you breathe.  It sounds weird when there are no breaks between phrases.  Depending on tempo, the break can be a quarter note or a 32nd, or something in between.
• Keyboard players—and I am one—tend to bunch the notes up together like a piano chord.  String arrangers voice chords with more space.
• Tried and true trick is to start with low strings, then move then higher as the song builds, and then add brass for drama.  Woodwinds are awesome for counter melodies and short solo lines.
 
Of course, you can do things completely differently: these are just some basic thoughts.
 
Good luck and let us hear what you come up with.

Konrahd is the man! Open voicings or wider spaces between the intervals. We also overlooked that orchestral players get very bored very quickly just sawing away on one note for 8 measures. There is usually a lot of interaction between the different instruments for color. And there is most definitely a lot more moving around note-wise other than just padding away. There is always a little bit of counter melodies and little fill riffs going on. DON'T OVERPLAY! just add some character to the parts. Start with just using cellos for an opening or 1st verse. The next verse, add the violas, saving the full violins for the chorus. This layering can make for very intense dynamics. Start listening closely to movie soundtrack material and you will get a wealth of ideas using these techniques.

Bristol_Jonesey
... Freeze the synth, not the track.

Not trying to derail this thread, but I'm wondering what you mean by this. I take you're saying don't use the "snowflake" button?

There is a site somewhere, I forget where but I saw them, that has midi effects, or a cal, that will separate the different parts for you. I want to say Keni, a member here, but I'm not sure. You have to buy them but it would make the separation of the different parts go much faster.
 
Maybe someone here knows about them?

To freeze a synthesizer instead of the track, open the browser, click on the synthesizer tab and click the freeze button on the synthesizer itself.