mysql database overload, please help

[jet rex]

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info of mysql server

OS: FreeBSD 7.2-RELEASE amd64
CPU: Xeon(R) CPU E3110 @ 3.00GHz (2 cores)
RAM: 4GB
HDD: 2x250GB raid1 SATA

my.cnf:
[mysqld]
user=mysql
#bind-address=127.0.0.1
port = 3306
skip-external-locking
myisam-recover
safe_show_database
skip-name-resolve
innodb_file_per_table
key_buffer = 2500M
net_buffer_length = 128K
myisam_sort_buffer_size = 32M
max_allowed_packet = 4M
table_cache = 2048
sort_buffer_size = 16M
read_buffer_size = 1M
thread_cache = 50
query_cache_size= 128M
query_cache_type= 1
query_cache_limit = 4M
# Try number of CPU's*2 for thread_concurrency
thread_concurrency = 16
# flush all tables every day
flush_time = 86400
max_connections = 400
max_connect_errors = 1000
max_join_size = 32M
join_buffer_size = 1M
max_user_connections = 300
wait_timeout = 120
interactive_timeout = 3600
innodb_buffer_pool_size = 5000M
#innodb_log_file_size = 256M
innodb_log_buffer_size = 4M
innodb_flush_log_at_trx_commit=2
innodb_thread_concurrency=8
innodb_flush_method=O_DIRECT
tmp_table_size=128M
long_query_time=1
log_slow_queries=/home/logs/mysql_slow.log
log_queries_not_using_indexes=1
#log=/home/logs/mysql_all.log

largest 20 files:
-rw-rw---- 1 mysql mysql 1425124660 Nov 2 14:03 post.MYD
-rw-rw---- 1 mysql mysql 376264704 Nov 2 14:00 xperience_stats_changes.MYI
-rw-rw---- 1 mysql mysql 348771044 Nov 2 14:00 xperience_stats_changes.MYD
-rw-rw---- 1 mysql mysql 324663296 Nov 2 14:01 postindex.MYI
-rw-rw---- 1 mysql mysql 292878261 Nov 2 14:01 postindex.MYD
-rw-rw---- 1 mysql mysql 285981084 Nov 2 14:03 postparsed.MYD
-rw-rw---- 1 mysql mysql 196611180 Nov 2 13:45 pmtext.MYD
-rw-rw---- 1 mysql mysql 183098555 Nov 2 14:01 word.MYD
-rw-rw---- 1 mysql mysql 114419712 Nov 2 14:03 post.MYI
-rw-rw---- 1 mysql mysql 89029672 Nov 2 08:43 picture.MYD
-rw-rw---- 1 mysql mysql 52694016 Nov 2 14:02 reputation.MYI
-rw-rw---- 1 mysql mysql 43454160 Nov 2 08:54 postedithistory.MYD
-rw-rw---- 1 mysql mysql 43037320 Nov 2 14:03 user.MYD
-rw-rw---- 1 mysql mysql 42145764 Oct 27 22:38 paymenttransaction.MYD
-rw-rw---- 1 mysql mysql 39546100 Nov 2 14:02 thread.MYD
-rw-rw---- 1 mysql mysql 38496256 Nov 2 14:01 word.MYI
-rw-rw---- 1 mysql mysql 36760844 Nov 2 14:02 reputation.MYD
-rw-rw---- 1 mysql mysql 36632590 Nov 2 14:00 xperience_stats.MYD
-rw-rw---- 1 mysql mysql 27294720 Nov 2 14:01 thread.MYI