What settings / parameters can be tweaked to increase the amount of buffers available as to not go to a temporary file?

We are seeing many of these:

2011/10/21 14:01:24 [warn] 24547#0: *2408454 an upstream response is buffered to a temporary file /var/lib/nginx/fastcgi/5/69/0000017695 while reading upstream, client: 1.1.1.1, server: www.someserver.com, request: "GET /edit.html?id=6816841 HTTP/1.1", upstream: "fastcgi://192.168.1.5:9000", host: "www.someserver.com", referrer: "http://www.someserver.com/c.html"

I tried messing around with proxy_buffer settings, but not sure if I'm on the right track.

Thanks