Tips for getting good synth string sounds:
•If possible, have one real violin playing doubling the highest voice in the string line. For even more added realism add a cello and/or viola too. Sometimes if there's just an acoustic bass player hanging around, I'll just have them grab their bow and play along. The sound of even one real string blended in with the synths immediately doubles the realism to the ear. It's the resin thing!
•Layer some different string patches to take advantage of strengths and weaknesses of individual sounds. A patch with a great attack, for instance, can be layered with a patch with a great sustain. It often is beneficial to use two different manufacturers as source material (I often use Kurzweil and Roland). This trick, by the way, also works really well to get really good reverbs (layering two diffferent manufacturers).
•If you have really good solo string samples or sounds and know how to arrange, you can try recording your strings the old fashioned way: write out parts for 1st & 2nd violin, viola, cello, and bass and then record them one stereo track at a time.
Good luck!