That's a very poor formula for pi. It converges very, very, very slowly.
Try some of these links for some better formulas:
http://www.geocities.com/SiliconValley/Bay/9187/pi.htm
http://elephant.linux.net.cn/articles/pi.php?lang=en
Chudnovsky's algorithm would be a fine choice. You get 14 digits every iteration of the algorithm.