First of all, I want to say that I think way too much emphasis is put on height for serving. Yes, tall people like 6'3" or whatever can pound in big serves, it is largely due to leverage and technique than actual heigh above the net. I am 5'8" and have a great serve. Look at Henin-Hardenne. She's only 5'5" and has one of the most formidable serves on tour.
Ok, now that I have that out of the way, to answer the question. It is hard to tell you exactly what you can do for more spin and pace into your serve when we can't see what your serve looks like right now. Any single element that could be missing would improve your serve all around: proper toss, good knee bend, proper timing, good backswing, tucking tossing arm in, weight shift, loose wrist, shoulder rotation, and I could go on... See my thread "Want A Better, More Powerful Serve? (The 10 Step Program)" and see if any of that helps you.
I do think a topspin serve would be very important for you to learn, though, if you want to improve it. You can never go wrong when you got a kicker.
