DB2 - How to estimate proper BUFFERPOOL size?

Question

DB2 9.7. 8GB RAM on server. Currently BP has 500 MB (number of pages*pagesize) Database has issue with increased IOPS. So we think maybe BP should be increased. How to estimate needed size for BP? Is there some recommendation? Maybe I would like to put it on 1GB. What can be negative effect? Is it always greater better for BP?