1 out of 25 sounds good to me. Not all people that visit your site decide straight away to sign-up. Check how long they stay on your site and if they do click on the banner (outgoing link counter). But wouldn’t complain with 1/25
I find that aprox. 50% of clicks result in a download,
aprox. 20% of downloads become signups.
On the low side a minimum of 10% of signups become reals. On the high side 30% become reals. It seems to be between the two most of the time.