Best approach to realistic snowfall and accumulation

I was wondering what would be a better approach to a realistic snowfall event, and having the snow “accumulate” on the objects it falls on. I could do a particle system, but I don’t think particles (using a snowflake object as particles, Cycles render) can pile up on each other. I could do a huge array of one or more snowflake objects, but it would require a lot of computer power and wouldn’t look as good as particle objects falling. Any ideas, or link to a tutorial would be a help. This would be for an animation.