MySQL 5.1 InnoDB Configuration / 24GB RAM - bi-xeon high load

i'm running a facebook app which currently has 300 - 600 concurrent users (and growing). To get the hardware ready for growing i changed my i7 / 12gb ram / 2x 80gb intel x25 ssd's (debian 5.0 / mysql 5.0 / 64bit) into a bi-xeon / 24gb ram / 2x 120gb intel 320 ssd (ubuntu 10.10 / mysql 5.1 / 64bit). now i'm facing the problem that the performance is worse than on the "smaller box". On both servers i've been using nginx / php fcgi to serve the content. i'm using innodb only, having Reads / Writes about 65% / 35%. Around 800 - 1000 qps but all Queries are simple and never join more than 1 additional table. All indexes are set and no individual query is logged in the slow log (> 2s). At the moment i have around 400mb of data (around 1gb with indexes) expecting it to double every month. I'd adore everyone who could give me a hint what to change to make it run smoother. The old configuration on the i7 box was like this (mixed myisam / innodb), performed pretty good up to 800+ users. old my.cnf key_buffer = 3000M max_allowed_packet = 128M thread_stack = 192K thread_cache_size = 8 max_connections = 400 table_cache = 8000 thread_concurrency = 16 query_cache_limit = 8M query_cache_size = 128M wait_timeout = 10 interactive_timeout = 10 connect_timeout = 600 low_priority_updates = 1 join_buffer_size = 8M read_buffer_size = 2M sort_buffer_size = 3M myisam_sort_buffer_size = 32M read_rnd_buffer_size = 4M innodb_buffer_pool_size = 3G innodb_log_buffer_size = 8M The new configuration on the bi-xeon box is like this (pure innodb), causing high load with 300+ users. Around 30 mysql processes sitting on the top of the process list. Disk I/O: avg-cpu: %user %nice %system %iowait %steal %idle 36.28 0.00 1.60 0.17 0.00 61.95 my.cnf key_buffer = 64M max_allowed_packet = 1M thread_stack = 192K thread_cache_size = 128 max_connections = 500 table_cache = 512 #thread_concurrency = 10 sort_buffer_size = 256K read_buffer_size = 256K read_rnd_buffer_size = 256K tmp_table_size = 32M max_heap_table_size = 32M query_cache_limit = 1M query_cache_size = 128M query_cache_type = 1 innodb_file_per_table = 1 innodb_data_file_path = ibdata1:1000M:autoextend innodb_buffer_pool_size = 16384M innodb_additional_mem_pool_size = 8M innodb_flush_log_at_trx_commit = 1 innodb_support_xa = 0 innodb_lock_wait_timeout = 50 innodb_flush_method=O_DIRECT innodb_log_files_in_group = 2 innodb_log_file_size = 128M innodb_log_buffer_size = 8M innodb_thread_concurrency = 12