I am running MySQL 4.1 on RedHat Linux. The box has 2GB of RAM, it has dual 2.8GHz Xeon processors, and /etc/my.cnf file looks like this.
[mysqld]
…
key_buffer = 512M
max_allowed_packet = 8M
table_cache = 512
sort_buffer_size = 32M
read_buffer_size = 32M
read_rnd_buffer_size = 128M
myisam_sort_buffer_size = 256M
thread_cache = 32
query_cache_size = 256M
I am running data mining process that updates/inserts rows to the table (i.e. I do multifield select on indexed fields, and if row is found, I update the data, if not I insert new row). Up to about 15,000,000 rows (1.4GB of data) the procedure was quite fast (500-1000 rows per second), and then it started to slow down. Now I have about 75,000,000 rows (7GB of data) and I am getting about 30-40 rows per second.