How to make CREATE INDEX in InnoDB table faster
RHEL5 ships with MySQL 5.0, unless you installed something else.  So I would focus on increasing these three variables.  They are not specific to either InnoDB or MyISAM.  

read_buffer_size        = #of Mb
sort_buffer_size        = #of Mb
tmp_table_size          = #of Mb


http://dev.mysql.com/doc/refman/5.0/en/server-system-variables.html#sysvar_read_buffer_size

http://dev.mysql.com/doc/refman/5.0/en/server-system-variables.html#sysvar_sort_buffer_size

http://dev.mysql.com/doc/refman/5.0/en/server-system-variables.html#sysvar_tmp_table_size

example :

read_buffer_size        = 16M
sort_buffer_size        = 64M
tmp_table_size          = 256M


Increasing the buffer_pool won't help you to speed up index creation. It is used for caching.

http://dev.mysql.com/doc/refman/5.0/en/innodb-buffer-pool.html

Also, if it were possible, I would also try to put the tmpdir on a faster disk.