Yes, this is about normal. My 3560 is using 3777424 of 8380416 (45%), and this is lightly used switch. I don't support the switches in terms of memory allocation issues, but from what I've seen above 80% is considered high. Of course, what you need to be wary of with I/O memory is to see if the switch stops forwarding traffic or if you start seeing buffer or memory allocation problems.