If you want to be able to tune MySQL to utilize multiple cores, you need to upgrade to MySQL 5.5.
According to MySQL's Whitepaper "What’s New in MySQL 5.5 Performance and Scalability" from December 2010:
Control of Background I/O Threads – Users now have two new
configuration parameters for all platforms, innodb_read_io_threads and
innodb_write_io_threads that allow for the setting of the number of
background threads used for read and write requests. This helps users
tune and scale their MySQL applications on high-end, multi-core
systems Control of Master Thread I/O Rate – Users can now configure
the overall I/O capacity
The parameters mentioned are innodb_read_io_threads and innodb_write_io_threads.
These settings do not exist before MySQL 5.1.38. In fact, only the MySQL Plugin allows for these settings in MySQL 5.1. It is included in MySQL 5.5's native InnoDB.
Perhaps this Blog Post with give you more insight into InnoDB Scalablity for Multiple Core Servers.