I came across really wonderful online vectorizing service: vectormagic.com and very impressed by results it creates. This graphics reminds me great old video game Another World. Of course, automatic vectorization simplifies snowflake structure and reduces number of colors, but it looks really nice, at my taste.
Vector files are free for non-commercial use (Creative Commons Attribution-NonCommercial license).
Real snowflakes in album Snowflakes and snow crystals.
Also i wrote article about snowflake macro photography.