Printing a block in node.tpl.php
I think module_invoke() is correct and you can simply call that in your tpl file. However,

use 

print $block['subject'];

print $block['content'];


instead of 

print render($block);


Also

D6 and earlier:

$block = module_invoke('module_name', 'block', 'view', 'block_delta');
print $block['content'];


D7:

$block = module_invoke('module_name', 'block_view', 'block_delta');
print render($block['content']);


Refer 


http://drupal.org/node/359021 
http://drupal.org/node/26502


for more some more help.