I'd like to share my first actual success on mysql tuning, after having spent a couple of days reading everything I could about the matter (and still waiting for the books to arrive).
From what I've seen a very important point on DB optimization is the right table design, followed by the right queries and finally optimizing DB parameters. Since I don't know enough yet about optimal DB design I'll skip that phase (tho I'll definetively accomplish it during the next weeks/months) and examining some queries.

After enabling log_slow_queries, one of the first queries popping out continuously was the following:

SELECT *, inet_ntoa(src_ip) as aux_src_ip, inet_ntoa(dst_ip) as aux_dst_ip FROM event_tmp order by id desc limit 1;