Instead of changing the default bufferpool parm, it is best to change the bufferpool size for each bufferpool directly. You can do this with the alter bufferpool statement, or use the Control Center.
Remember that the bufferpool size is expressed in pages. On your 640 MB machine, I would allocate a total of about 256MB to your DB2 bufferpools (total amount of all bufferpools for all active databases). If your only bufferpools have a 4K page size, set the number of pages to about 30,000 for each of the two bufferpools (one in each database).