increasing server_names_hash_bucket_size

[rkmr.em at gmail.com]

hi
i have configured nginx for multiple virtual servers, when i start
nginx, i get this error:

2008/02/26 13:44:31 [emerg] 18598#0: could not build the
server_names_hash, you should increase server_names_hash_bucket_size:
32


if i increase it to 64 or 128 the error is gone and it nginx starts working.
now how much to increase this ? 64 or 128 or even higher? is there any
performance effect because of this?
thanks