"Where is a good place to start?"
If you are happy with the old string, find out what material and strand count it is and get a duplicate.
Strand count depends on draw weight and string material. With many of the "fast flight" materials, you can go ridiculously low...some folks say there's an advantage to these tiny diameter strings, I haven't seen any real benefits myself (I've made/shot them). You loose durability and gain stretch/creep with a low strand count.
I like Dynaflight '97 and 8125. I've had good results with the new 8190--very low stretch, very low creep, very durable. Strand count...my suggesstion would be (with your bow weight):
Dynaflight '97 12 strands
8125 12 strands
8190 16 strands
450+ 10 strands
Fewer strands will be strong enough, but you won't normally gain much if anything.
Chad