I've a vague feeling that graphite bows would tend to be lighter than wooden ones if anything. No idea where it came from though, and I'm hardly an expert. I'd say find a decent shop and experiment a bit. It depends so much on style, what your arms and stuff are like, just what feels comfortable. People who are really serious shell out absolutely masses of money for bows by particular makers, but then people buy stradivari as well. Most of us aren't made of money.