If you're counting bytes you'll need to account for TCP/IP overhead in addition to HTTP overhead. My memory (which is probably wrong) is that TCP/IP overhead is in the neighborhood for 46 bytes per packet. Since you're dealing with small data, you're probably not splitting packets.

Since the overhead of setting up (and tearing down) a TCP connection is 7 packages (3 to open, 4 to close), it's to your advantage to keep the connection alive. (See Connection: Keep-Alive and Perl for details.)

In a somewhat similar situation a while back, on a machine 1/2 as powerful, but doing a lot of work on the back end per request, we were able to handle 150 simultaneous connections, though they were polling every 5 seconds.