If you are having a weird problem with your payment modules not saving the configuration when you edit them via the admin, try this to see if it fixes it:
in admin/modules.php
find:
PLAIN TEXT
PHP:
while (list($key, $value) = each($HTTP_POST_VARS['configuration'])) {
and replace it with
PLAIN TEXT
PHP:
//while (list($key, $value) = each($HTTP_POST_VARS['configuration'])) {
foreach ($HTTP_POST_VARS['configuration'] as $key => $value){
More Reading:osCommerce [...]