Actually I would really recommend to implement possibility to limit axis of rotating for particle billboards. Add at least vertical billboard rotating only around vertical axis (usable for torches and candles) as I suggested in my example:

http://www.esenthel.com/community/showth...p?tid=4700

Why? It is performance booster for free. I can aeasily achieve similar result with 1 particle instead of 40-60. For small flames like candles and torches it really makes sense, because in typical room, I have dozens of them. Big flame is indeed something different.