Hi there, I just joined and this is my first post, so be kind!
It's important to familiarize yourself with the ranges and techniques of the various string instruments, but it's also important to think about how they fit together in terms of voicing. For instance, independent lines in the double basses are uncommon, but not unheard of, and the first and second violins can play the same line, or you could have the first violins play one line and the second play another, or have the two parts play divisi. The possibilities are almost endless!
There are lots of great orchestration books, with Rimsky-Korsakov's being one of the great classics: http://www.amazon.com/Principles-Orchestration-Dover-Books-Music/dp/0486212661/ref=sr_1_1?ie=UTF8&qid=1352780499&sr=8-1&keywords=rimsky+korsakov+orchestration
However, I highly recommend Samuel Adler's Study of Orchestration: http://www.amazon.com/Study-Orchestration-Samuel-Adler/dp/0393958078/ref=sr_1_1?s=books&ie=UTF8&qid=1352780526&sr=1-1&keywords=samuel+adler+orchestration
It was our textbook in my orchestration class in college, and I still refer to it frequently when writing for any instrument. The section devoted to strings is very readable and will teach you a lot, including the pitches of each string and how they can be fingered -- this may give you ideas in and of itself!
I see this thread is a few months old so you may have already completed your project, but I hope this is helpful just the same!