Here's a chart of needles:
http://www.schmetzneedles.com/Schmetz_Sales_Guide.pdf
http://www.schmetzneedles.com/pdf/ABC_Brochure.pdf
I've recently gotten to love using topstitching needles. When you read the chart it seems like they're the best of all worlds.
Here's a thread guide that maybe can be useful also:
http://www.schmetzneedles.com/sizingguide.html