You seem to be running 32-bit Linux and have 349 concurrent connections.
If you have not changed STACK_SIZE in LinuxThreads and built the binary 
yourself, LinuxThreads is quite likely to steal a part of the global heap for
the thread stack.

key_buffer_size=536870912
read_buffer_size=4190208
max_used_connections=373
max_connections=425
threads_connected=349
It is possible that mysqld could use up to 
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_connections = 155068
0 K
bytes of memory
Hope that's ok; if not, decrease some variables in the equation.