Two things catch my attention

- MEMORY
- CPU

I have addressed these issue in past posts

- http://dba.stackexchange.com/questions/17092/is-the-cpu-performance-relevant-for-a-database-server/17098#17098
- http://dba.stackexchange.com/questions/10899/insert-heavy-innodb-table-wont-use-all-my-cpu/10900#10900

Please follow suggestions centered around InnoDB as multiple core engagement is only available in MySQL 5.5's native InnoDB as well Percona's XtraDB.