[UnderHost]

Used MySQL memory = key_buffer + max_connections * (join_buffer + record_buffer + sort_buffer + thread_stack + tmp_table_size)

Notice the max_connexion and the multiplier.
connexion increase = memory usage increase too.

Notice key_buffer
for a given memory :
more you add mem to key buffer, less connexion is
less is key buffer, more connexion is

If you change one of these settings for a high value, you system may swap.
If you system swap, try lot decrease these values

Also, about table_cache :
Increasing the size of the table cache may really help you.
But you must be careful not to make the value too large. All operating systems have a limit on the number "open file pointer" (sorry in french it is called pointer, maybe descriptors is the good translation) a single process may have.
If MySQL tries to open a lot of files, the OS may refuse it and MySQL will generate error message in the error log.