Smilies do not affect performance to any significant degree if your server is well configured. The load comes from the updates of the chat content by each client. The smilies do not reload so are not a problem.
Make sure you have "expires" on though for images, js, swf and css files. The browser will not have to fetch the images again for each reload then. It can be as low as an hour or two, as long as it's on you save yourself a lot of bandwidth and lower the number of HTTP connections.
http://www.askapache.com/htaccess/apache-speed-expires.html
http://www.askapache.com/htaccess/apache-speed-last-modified.html
Also try to have "keep alive" disabled as each HTTP connection stays connected longer than needed if it is enabled which tends to increase load on heavily utilized servers. But you mileage may vary.
http://serverfault.com/questions/73812/should-i-activate-keepalive-in-apache2