If you can run mod_gzip, go for it and don't worry about cgi_buffer - it'll be a waste of resources.
mog_gzip can compress your HTML, HTM pages as well - you specify which file extensions should be compressed if supported and also on a browser by browser level as well.
You can also tweak the compression setting - cgibuffer uses 4 but 1 is actually better in this circumstance because the balance between cpu time and compression is just not worth the load on your server.
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